This is something that is not at all settled IMO, assuming you're referring to cash game staking (or long-term tournament staking with makeup).
One-off (i.e. non-long-term) tournament staking is pretty clear; the wins/losses are allocated between the player and the backer in accordance with their contract (discussed in
the tax sticky). The idea here is that when you've got backers or stakers, the gambling wins and losses are realized
by them, rather than by you with your settling up with them being a distinct event. Note though, that this treatment of one-off contracts still has both the player and the backer realizing sessions with the same frequency they would if there were no backing.
To make this analogous to a long-term contract, let's say one where you play every day with someone else's money and only settle wins/losses with monthly payouts, you might argue that you'd just only realize a win or loss once per month during this settling. (And this is what I'd personally do if I were in this situation, though I'd want to check with my tax professional.)
However, there may be issues with this, as entering such a long-term contract could, under some conditions, just be seen as a way to allow an amateur player to net sessions. Example being if you were to "back" me by providing me money to play with, but having me keep 99.9% of winnings and eventually owe you 99.9% of losses on a monthly (or even less frequent) basis. I'd effectively get the same EV from my year of play that I would unbacked, but I'd get to lump it all into one session. It may be seen as an artificial way to delay realization of wins and losses.
So there is a potential exploit here if it
were to be the case that long-term backing deals lead an amateur to only realize sessions on the dates of settlement. I've never read anything about this and I doubt there is any real of implied IRS precedent, they're probably not even aware of this possibility. I don't know that I'd want to be the test case, but I also don't know if this is really worth worrying about.