I have a Vigor 2925 and would like to know if it's possible to either limit access to a period of time or limit usage for a period of time.

I currently have a download limit on my fibre connection of 50gb. However, with two teenage girls in the house, 50gb can go in a few days. I could just change the password and not tell them but would prefer to limit the useage rather than a complete cut off.

Here's a half-helpful answer. Yes, it is certainly possible, but I can't give you detailed instructions
You can create accounts in user management and give them a budget each. e.g. Teenage Daughter 1 - 2 hours per day. Or you can allow her to have wireless on, say 6pm-8pm and she has it off the rest of the night.