Indeed, it's a different thing for different people. For me personally it's around the amount of money I can earn in one day. Yes, it is still painful, losing the money you were working for for the whole day, but it's acceptable, it's not the end of the world, you know.
But that's about the money I can afford to lose without bad consequences. Talking about the money that I can easily afford to lose, than it's about a quarter of what I make per day, and that's what I would bet on the single die roll, you are asking in the OP.
That's a sensible approach that can work for you but is not universal. Obviously, for someone with a low income and a tight budget, who struggles to pay the bills (aka half of the UK right now after massive electricity rates hikes lol), a daily pay (or even just 25% of it) is probably too much.
Personally, for the sake of regular gambling, I'd define the "amount I can afford to lose" as excess (or leftover) money at the end of the month, after paying all bills and usual living expenses and also after making a regular contributions to saving/investment accounts.
But that definition and my risk tolerance would change for things like say investment opportunities with lower risk and positive expected returns. In such case I'd be potentially willing to risk any amount that won't cause a massive disruption to my life.