Would it increase/decrease over time if kept in an airtight container?
To answer questions about why I would be keeping it locked away:Considering building up a large amount as a gift for a friend before she moves.

Increase.

No wouldn't change anything but you should (I would) keep it in a glass mason jar or else it'll end up all dried out and harsh to smoke.

Okay, thanks.

Keep it in a dirty puddle of water.

Yes it does. It's known as curing. Look it up.

It also gives the bud a smoother smoke, and a better taste.

I'm not entirely sure if this applies to bud that's long since been harvested. Usually it's done right after the bud is properly dried.

Usually, curing marijuana in an airtight container in a dark place does increase potency and taste, but that time is window is usually shortly after harvesting and the initial dry.

Yes it does. It's known as curing. Look it up.

It also gives the bud a smoother smoke, and a better taste.

I'm not entirely sure if this applies to bud that's long since been harvested. Usually it's done right after the bud is properly dried.
Yeh, if you do it right but if you just toss some buds in a baggy and toss it in a drawer for a few weeks that isn't going to be very nice to smoke especially if you're opening the bag and adding more buds frequently...if stored properly (ie. in a jar) then yeh, it won't be a problem. If you just store it in a bag it'll get harsh dry and nasty....

Yeh, if you do it right but if you just toss some buds in a baggy and toss it in a drawer for a few weeks that isn't going to be very nice to smoke especially if you're opening the bag and adding more buds frequently...if stored properly (ie. in a jar) then yeh, it won't be a problem. If you just store it in a bag it'll get harsh dry and nasty....

keep in mind this guy is a flamer and he likes things harsh dry and nasty.