How much time do you have to spend alone? How frequently?
Depends what you mean by alone time. Actual alone time is probably a few hours each day because I get home from work before my partner does. But I spend a lot of time just hanging out with him while we each do our own thing in companiable silence (he's another introvert). I rarely find that draining, although I do enjoy having a bit of complete alone time.
What does the required amount depend on?
If I've overdosed on human contact (eg being at an all day meeting at work where I've had to do a lot of talking during the meeting and socialising/networking during the breaks) then I need a lot of down time to recharge. Also, going away on vacation with a group of people can be quite draining and I usually need to go on short walks by myself, or just with my partner, during the vacation to have a break from all the people. It also depends on how extraverted the people are. Extreme extraverts with loud voices and who never STFU are way more draining than low-key extraverts and other introverts. I also need alone time after I've been in crowds, like at festivals or a crowded mall.
What do you do when you are alone?
I usually go online or potter around the house. Online interactions are much less draining than RL contact.
Do you ever feel you have been alone too long?
Occasionally yes. A week without contact with anyone is too long but a couple of days is fine.