I've used Trusted Housesitters - https://www.trustedhousesitters.com
I've had good experiences, but I will say that general consensus is that 99% of sit go smoothly and the other 1% are absolutely batshit. It's 100% on you to vet the homeowner. The company takes no responsibility if something goes wrong.
For how long?
This year I spent 8 months housesitting. 5 sits total, with the longest being 3 months.
I also imagine getting someone to trust a new user of the app would be quite difficult.
Yeah, getting the first sit is typically the hardest. I personally got my first sit fairly quickly and I got my second sit before the first sit ended (i.e. before I had any reviews), so it's doable.