The literal answer to your question is 250, but I think what you want to know is how much you should raise it at a given time (or at least, that's what I would want to know if I asked a question like this).
For gathering professions there are 5 tiers (TERA seems obsessed with tiers):
- tier 1 are in areas <20
- tier 2 are in areas <30
- tier 3 are in areas <40
- tier 4 are in areas <50
- tier 5 are in areas <=60
The idea being that as you progress in gathering professions you'll be able to acquire higher tier nodes. If you aren't keeping your gathering up with your leveling you'll start failing a lot.
Each tier corresponds to about 50 skill points, so you'll want to make sure you're hitting that rate of improvement as you move through the zones, otherwise it'll become more difficult.
Having a high gathering skill doesn't provide stats on it's own, but successful gathering does provide benefits (such as increased stamina up to 135). These temporary (usually 10-20min) buffs make leveling significantly easier. However, if you haven't kept up on your gathering skills you'll find higher tier nodes hard to mine (multiple failures and even destroying plentiful nodes before you extract everything out of them), so you lose out on said buffs. Thus, while having a high skill doesn't directly denote a buff, it indirectly allows you to receive buffs.