There's a huge gray area regarding tax liability accruing to free play, but the most important facts are:
1. You owe taxes on ALL gambling winnings (not just those that generate a W2-G). From an accounting standpoint, that would be however much actual cash your $50K of free play puts in your pocket.
2. Casinos don't issue W2-Gs for nonmonetary prizes, and technically, free play is nonmonetary. Another reason they don't do that is that $1 of free play is worth less than $1, and they can't even say exactly what it is. So it would be inaccurate to issue a W2-G for the face amount of the free play--or any lower estimate.
3. The fact that a W2-G is or is not issued has no bearing on your ultimate tax liability.
4. The vast majority of gambling winnings go unreported. Enforcement is minimal and sporadic.
As far as requirements to play off the free play in a day or whatever, there are no set rules. One time I won 10K in free play at the Palms and had three days to play it off. I'm sure that CET has its own way of doing things.
One final note, should you ever win such a prize: almost certainly, you would be able to play the free play on video poker as well as slots. The difference between playing 50K through a VP machine and playing it through a high-denom slot is anywhere between $2000 and $5000. Plus, a bad run that leaves you with half of your money--or less--is much less likely on a VP machine.