Given the arbitrary nature of your Experienced/Veteran definition (per the discussion in comments), there are three methods for determining this that I see, both of which require you to make judgment calls:
1) After determining how much XP you are aiming to give out in an average session, determine how long you think a player should play a character (either in number of sessions - egs. 20 sessions for 'experienced' 40 for 'veteran', or in RL time - egs. Six months for 'experienced' a year for 'veteran') before attaining a given threshold. The math, at that point is fairly simple.
If you're looking to give out 10-15 xp per session/character, run game weekly, and think that an 'experienced character' should take about six months of regular play to build, then you're looking at 200-360xp for an 'experienced' character. (20-24 sessions @ 10-15 xp per session.)
2) Another method for determining this is to determine what the final stats would look like, to you. Build a handful of characters you consider 'experienced' and then figure out what their starting builds looked like. Then it's simple arithmetic to calculate how much XP is required to buy up their stats. You can use character age at the end of the life path system as a decent marker.
3) The final method I would recommend is to use the Battletech pilot experience level metrics, but assume that the characters attaining those skill levels aren't played by munchkins. Decide on a ratio of Btech-relevant skill xp versus Btech-irrelevant skill xp. As you note in the comments, you can come out of chargen with Veteran combat stats, but atrophied social/technical/adventuring skills. Mechjock Savants aren't very common in the btech setting, so by building a 'green' starting character and then spending xp on that template until you attain regular/veteran/elite stats, you can find out how much XP your version of green/regular/etc has.
e.g. Let's say that after starting build as green you require 200 xp to attain 'Regular' combat skills. Let's also say that you conclude that no more than half of a given character's xp should be spent on combat skills. This suggests that a 'Regular' (or 'Experienced') character is one that has at least 400 xp under their belts. If you decide that one third of the xp should be spent on combat skills (or, more accurately, that two-thirds of the xp should be spent on NON combat skills) then you're looking at 600xp for a 'Regular' character.
Many players wind up building ace mechjocks that can't talk their way through an average conversation at a bar without blowing half their edge pool, but if you insist on your NPCs being more well rounded, you can smooth out the wonkiness brought on by munching out the starting build.