Depends on the slot you choose tbh, how big or small your spins are... and luck.
I usually don't bother with casino offers but I went for the WH deposit 10 get 10 offer over the weekend. Wagering was x20. I find Gonzo's Quest pretty reliable for bonus rounds so I stuck with that. At the end of wagering I was £130 up, but the maximum they let you take was £100. I was pretty chuffed!
I don't quite get what you mean when you say you've bet £50 on the right slots, though. You mean you've been doing spins and wagered £50 so far? Which slot? Not hit any bonus rounds? The best you can hope for is to hit enough bonus rounds to keep your balance topped up enough to get through the wagering - but it's not risk free of course.