I think every writer is seeking different things from a beta, so if you want to be a beta, make sure you ask that writer what they’re after.
Do they just want a read through on spelling and grammar?
Are they after help with plot?
Are they looking for someone to just comment on their emotional reactions to the story?
Do they want critique of their entire story, or are they just looking at particular parts of it, like how certain characters relate, or how certain plot points pan out?
And let them know what you feel comfortable providing as well. Spelling and grammar might not be your strong point, but you might have really great insights into how a story should flow.
Every writer-beta relationship is different!
One thing I would stress though, is that if you need a certain amount of time to beta read, try your best to stick to that, whether it’s a few days or a few weeks. Most of us writers are really desperate to get that feedback back from you!