I've been reading fanfics for years and I read a lot in general.
With shows/books/movies I know well (i.e. those I'm offering my services for here LOL), I can do general beta services, i.e. check for characterization and story flow, point out inconsistencies etc.
If you write a story in German, I can also do grammar and spelling beta.
Or, if you write for a fandom set in Germany, I can check if your story fits with life here, our culture, the way we Germans act/think. I've done that for Yuletide, too.
I will not alter your text (you're the author, not me) but put in remarks for odd phrasing or give you general feedback on where I think your story might be lacking something or what I think is really good.
I don't want to rip your story apart! You wrote it because you want to tell this story, I respect that. And as a beta I see it as my job to offer constructive criticism to help you make it better where it might be lacking something.
I try to get beta-jobs done quickly, but I work full-time and I have my own hobbies. So depending on the length of your text, give me a few days. I want to do a good job for you and not have to rush through it. If it's just one chapter or something up to 2000 words I should usually be able to do it within a day, longest two if life is busy. Of course throwing stuff at me over the weekend is helpful as I have more time to tend to it then and you can expect it back sooner than throughout the week.