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When I was still married this was a the juggling act that we had to preform with nearly every major holiday... My ex-wife and I both came from split families so every occasion meant making appearances at 3-4 family functions, and very often having to carefully and delicately manage expectations for each.

What I found most helpful over the years was making an effort to address schedule conflicts early on in the planning stages. Just sort of making the various family factions aware of when the other factions were planning their events, which allowed them to adjust plans a little.

Obviously you can't always expect everyone to play nice and be all that accommodating, but by informing them of prior engagements before they've set a time for their event they usually will be. In the worst case, they'll understand that you will have to leave early or show up late.

In your particular situation, being somewhat new to your girlfriend's family, it's good to put your best foot forward, but now is also a good time to start setting precedent. More or less, be upfront about your prior engagements and family obligations. You may even find that her family respects you more for it in the long run, it shows that you're responsible and that you're inclined to respect your parents wishes (parents like that sort of thing.)

So... TL;DR

Setting a particular time limit shouldn't be perceived as rude in most cases and try to give people the opportunity to work out a mutually beneficial event schedule.

Years from now if you opt to settle down with this particular girlfriend and have children... You'll be the keeper of the grandchildren and they're likely to become much, much more accommodating.