Unlike Flora, I prefer to beta a finished fic, or at least one in pretty final draft. However, I can do it fairly quickly (usually = inside 24 hours), unless the fic is very long (over, say, two posts' worth). I've asked for a bit of notice mostly because I expect to be fic-wrestling myself at the time.
I cannot stop myself from commenting on SPaG and word choice, even if that's not what you ask me for. However, if SPaG errors are really frequent, I might resign and send it back. Ie, spell-check is your friend, as is a good dictionary, and so is careful re-reading, since spell-check alone isn't enough. Allowances made for non-English speakers (I've beta read for quite a few, and yes, they do come back), but please try; the cleaner a text is the more likely I am to be able to make helpful comments on other stuff.