Hi OP. Currently our cleaner is arranged via an agency, which has taken a lot of the stressful people side of things out of it (she turns up, we say hi and get on, but I don't need to ask her to do things because the agency owner sat down with her before she started). At one point she was unwell for several months, the agency sent substitute cleaners and one was really bad (partially mopping a dirty floor before hoovering, then partially hoovering and getting damp dirt everywhere) and the agency owner had the awkward conversation to move her on, not us.
In terms of your questions:
Is every week the norm?
We have once per fortnight for 2.5 hours, 2 bed, 2 bath, kitchen, living room and stairs (we stay in the third bedroom to keep out of her way, and clean that ourselves).
What would you expect to be covered over the course of 2 hours every week?
Depends what you want. Ours does the above in the 2.5 hours and everything is spotless. She doesn't do windows, we do those, and doesn't go inside cupboards, microwave etc. She cleans everything that's on show / out and dusts / hoovers underneath things but doesn't, e.g. get all the seat cushions off the sofa.
Do you provide cleaning supplies, or do they tend to bring their own?
Provide our own.
Do you provide a list of what you would like covered for that week, or do they go ahead and just do what they think needs done?
I agreed a regular list of 'every fortnight' items and sometimes when she's running ahead (we have no pets, no kids, very tidy house) she does a few extra bits.
Would cleaning inside of windows be covered?
IME, no. That comes under 'deep clean', not unreasonably, as to do it properly takes ages.
I started by having a one-off two day deep clean three years ago, following some building work, and was so impressed by how much better she was than us, and how much time and stress it saved, that we fell into a fortnightly pattern. She's definitely in my top five favourite people!