This question is a well deserved one, and I will answer it with a mixture of whatever I can find/remember in the book and my suggestions for house rules. I will endeavor to make it clear where the book starts and ends and what is simply an "educated guess" I have made.
- First an easy one, you didn't specifically ask it but as an easy first step I will mention that it is pretty clear when you get one of the three types of Reserve Fate Points. This type is the NPC compel type. The book says:
When an NPC earns fate points the same way that a PC does (someone invokes one of their aspects), you concede a conflict on behalf of that NPC, or you accept a player-driven compel—those fate points go into the reserve. (page 54).
- This is where it gets a little harder. The book does not seem to say anything about when or if you should lose your reserve Fate Points. The only thing it does say about reserve Fate Points is that it "starts at 0" (page 54 again). as far as PC Fate Points are concerned the book says this: "If an issue stretches beyond a single session of play, you start the next session with however many fate points you had at the end of the last one. It’s only with the advent of a new issue that you reset to your usual starting allotment of one fate point per character aspect" (page 53). This leads me to mention a couple options I can think of in terms of house rules.
The first one is that you lose reserve Fate Points if you do not use them by the end of the session or issue, issues can span 1 session or up to 2 or 3 even.
The second is that you get to keep them forever, this seems silly and open to letting a GM hoard an ungodly amount of Fate Points.
The third is sort of a hybrid, start with suggestion number one, with some exceptions. If your PC's have done something, say in the last 10 minutes (or whatever time amount your group as a whole finds reasonable) that increases the Reserve pool at a time when it is not very reasonable to expect the GM to be able to use them, the GM gets to keep those Reserve points for the next issue/session.
Alternatively, you can keep any reserve Fate Points for exactly one session/issue beyond the one you receive them in, in order to give you time to use them, but not time to hoard them excessively. This would simply require you to mark any Reserve Fate Points you have left at the end of a session, and keep them in a separate pile from the ones you receive next session (maybe have a card or strip of paper with writing to remind you) and nothing says you can't use up the ones that are gonna expire at the end of the session/issue first.
- Now I will attempt to answer borrowed hardware. This one is in the book at least in part, but it seems at the very least scattered around. for starters I saw this reference to go to page 139 to see if anything extra was mentioned on that page."Players pay for hardware they borrow the same way they pay for inventions they create, by increasing the GM’s fate point reserve (page 218).
Going to that page eventually led me to this:
The other payment for the invention is an increase in the GM’s fate point reserve. Add fate points to the GM’s reserve equal to invention’s quality. This is equal to the number of the invention’s stunt benefits minus the number of Bugs. These fate points come from the slush fund, not from the players. See Aspects and Fate Points (page 54) for more on the GM’s reserve (on page 145).
This rule is for inventing something, but seems to be at least heavily implied to be used for borrowing tech from your faction too. As for how many times it is added into the reserve, it seems to me that it should only be added once per session at most (possibly once per each issue, but this could be too long to get fate points again), due to the fact that the amount of Fate Points being given is based on the quality of the invention/borrowed gear. That being said, I could technically see it being paid for every scene it sees use, but definitely not any scene where it is not being actively used (or at least any scene where it is not useful).
- Now for Mega Stunts, these are kinda tricky. the first mention of them says
"If a PC with mega-stunts has more than five stunts’ worth of benefits, add one fate point to the GM’s reserve for every benefit in excess of five. See Mega-stunts (page 76) for more on what this means and how mega-stunts work." (page 54 again). On page 76 mention is made of weakness and cost, for this question weakness is mostly irrelevant, so I will mention cost.
A cost is a minor cost the GM can introduce by compelling your concept aspect. It’s always a minor cost, but the exact form it takes depends on the PC’s concept aspect, the circumstances, and common sense, and can differ from one instance to another. When writing the mega-stunt on your sheet, indicate a cost by writing “but at a cost” after the benefit. When the GM initiates a compel that stems from a weakness or cost, the player can refuse it, as usual (page 77).
The fact that you can compel the cost or weakness of a Mega Stunt is already intended as part of the cost. Because of that it looks to me like you should only get any Reserve Fate Points from these Mega Stunts once every session (or issue, but this, again, might be too long a time).
I also think it is worth mentioning that when a Player takes a Mega Stunt for the first time in the middle of a game, the GM should immediately add in the Reserve Fate Points that they are owed from it.