I can beta for any number of circumstances:
1. Grammar, wording, spelling and anything pertaining to technical writing aspects
2. Creative beta work, in addition to the one above, offering plot advice, and what I believe to be weak and strong points of the story.
I'm not sure how quickly I would get back to each individual author, but I'm on fanfiction.net very reliably, except in the case of a personal emergency such as an overload of schoolwork or personal crisis.
If you ask me to do more than beta for grammatical problems, or I spot something that really really bothers me in your story, then please listen to my advice. You applied for a beta for a reason, and if I feel like I'm being nothing more than a glorified spellcheck, then what do you need me for? Betas are human for a reason.
I'm not quite sure what my betareading style is, since I've never made an honest attempt to beta anyone else's work before, but I have edited other essays and writings in classes. I tend to print out a hard copy and write all over it. Then i give general impressions and advice at the bottom. However, I may have to work something out for purely online beta work.