There's no time limit, you don't need to worry about it being unavailable. The longer the game goes on, the more UFO invasions will occur and the more panic will rise, so that is a concern, but it may well be worth it if your squad is not up to it yet, in terms of weapons and armour.
Spoiler, not sure if you'll want to know this for your first playthrough, but it's useful to know: The alien base mission will reset panic of all countries (apart from those who've abandoned XCOM), to the lowest rating, so it may be worth delaying it until you have countries at max rating, gearing up and leveling soldiers, then doing it to reset panic. Just be careful not to wait to long as you'll lose them.