That's an impossible question to answer accurately. Could be 3 days, 3 weeks, 3 months or even 3 years. I think one thing that will affect how long it has is how you drive it. If you keep forcing it into gear by banging on it, obviously it'll have less time. If you baby it you might get more out of it.
Keep an eye on your trans fluid also. Make sure that it stays fresh and clean. When you drain it keep an eye out for sparkles, the more sparkles you have means that things are getting really bad.
By the by, have you drained the fluid to see if you have any metal flakes in the fluid already?