Pretty sure it's not ruined, but ...

I made a Hefe last week using Wyeast 3068 and those guys are active! Got everything into my plastic bucket, lid on, and the next day the lid was pushed up and off with Hefe flowing down the side of the bucket. Time to get me a glass carboy I guess

Obviously it was exposed to air, but I'm pretty sure it's not ruined, since it was only for a handful of hours early on in the process and I'm going to rack to secondary this weekend. But it got me thinking, just how much air can you expose the wort/beer to at this stage and still get a nice finished product?

Anyone have an idea?

It's not air exposure that's bad, it's the possibility of something in the air falling in and infecting your brew. In your case, it was actively blowing off and it's not very likely anything bad happened so don't worry about it.

Using a carboy instead of a bucket won't really change anything, either way for very active fermentations you want to use a blowoff tube instead of a simple airlock.

The funny thing is, I did use a blow off. Maybe I need to just get a bigger tube!

It's nothing to worry about- I have about 20L of deadspace and had krausen blow its way through the airlock hole.