Is this strategy for extending Time Stop via the Extended Spell metamagic valid?

Question

My party is level 17 and one of my PCs is a sorcerer.

He discovered a combo using the extended spell with time stop to just freeze time. Time stop lasts a minimum of 2 rounds or 12 seconds so by using 13 sorcery points he could freeze time for 24 h. He could then use 8 of those 24 hours to take a long rest regaining his sorcery points and the 9th level spell slot. He could then do whatever he wanted and just before time unfreezes he could recast it, rinse and repeat over and over again.

Unless he affects another creature, he can keep time frozen for as long as he chooses. I'm not so worried about the potential balance issues of this because I can introduce scenarios and enemies specifically tailored to counter this ability (homebrew if you were wondering), but I do wonder if this freezing time strategy is viable by official and Adventurers League rules?

Answer 1

You can use only one Metamagic option on a spell when you cast it, unless otherwise noted.

The phrasing may seem unclear but you cannot use several metamagic options and neither can you use the same metamagic option several times on the same spell.

Here lead rules designer Jeremy Crtawford his intent for the rule on twitter:

The intent is that a sorcerer can use a Metamagic option once with a spell, not the same option more than once.

Additionaly, Time Stop is not even eligible for Extended Spell :

When you Cast a Spell that has a Duration of 1 minute or longer, you can spend 1 sorcery point to double its Duration, to a maximum Duration of 24 hours.

Time Stop is instantaneous. Probably that the idea behind is that it actually stops time and the caster is just "out of time" for 1d4 + 1 turns

Answer 2

The text of Extended Spell, emphasis mine:

When you Cast a Spell that has a Duration of 1 minute or longer, you can spend 1 sorcery point to double its Duration, to a maximum Duration of 24 hours.

The text of Time Stop, emphasis mine:

Time Stop

9th level transmutation

Casting Time: 1 action
Range: Self
Components: V
Duration: Instantaneous
Classes: Sorcerer, Wizard

You briefly stop the flow of time for everyone but yourself. No time passes for other creatures, while you take 1d4 + 1 turns in a row, during which you can use actions and move as normal.

This spell ends if one of the actions you use during this period, or any effects that you create during this period, affects a creature other than you or an object being worn or carried by someone other than you. In addition, the spell ends if you move to a place more than 1,000 feet from the location where you cast it.

Since the duration is less than one minute, by any interpretation (the incorrect interpretation of 1d4+1 six second-ish turns, or the correct interpretation of instantaneous) it cannot be extended.

Answer 3

You can only benefit from a long rest once every 24 hours

The long rest rules state that

A character can't benefit from more than one long rest in a 24-hour period

So even if metamagic could stack and Time Stop was a valid target for Extended Spell, no time would be passing because time would be stopped. If it had been less than 24 hours since the previous long rest, the Sorcerer wouldn't be able to long rest while time is stopped.

Answer 4

The only way I can think of to extend time stop is to cast it twice. The 2nd time after the first one expires. And the only way to do that is to add 2 levels of fighter to your build and cast it once with your action then when it expires once with your surge. But it's a far cry from what you described.