Some more details, to complement Dale M's excellent answer. Note that this is general advice, as mentioned in the site disclaimer. For specific advice, ask a legal professional.
Is it fair to assume that most of those funds will be used over time
and I can use those funds? Are there any rules of thumb on this?
As Dale M explained, you can use those funds like any other money you have. It's yours for the time being. However, customers may ask for it to be refunded, in which case you probably will have to give it back (whether and how you can limit that depends on specific rules in your jurisdiction).
Or, perhaps, users forget about their funds in which what is my
responsibility to notify them about that?
Again, depends on jurisdiction and consumer protection laws, but usually there is no obligation. Still, it would probably be very bad for your image to let customers silently forfeit their money, so you better inform them.
Do I need to put those funds in a type of "escrow" account to be sure
it is available for them should they ask for a refund?
No, but you must make sure you have enough liquidity to satisfy any refund requests. If you don't, that is (usually) not a crime, but if you cannot refund, you will go bankrupt.