1.5 hours is do-able if:

1) Your inbound UX flight is not late

2) You know how to power walk well. UX is in terminal 8. You will need to walk past T7, T6 and T5 to get to TBIT.

There are two ways to get there. You can stay airside from T8 all the way to T6, exit at T6 and then walk to TBIT. Or you can exit at T8 and walk all the way along the curb to TBIT. Or, you can exit at T8 and hitch a ride on the Bus which will go past T1, T2, T3, T4 and then TBIT.

At TBIT you will need to re-clear security. Security at TBIT is hit or miss. But, I've never spent more than 30 minutes in line.

I personally HATE the bus. Too crowded sometimes. Too many lost passengers that delay the bus by asking the driver questions. Too many passengers delaying the bus dragging their life's possessions on to the bus.

It's about a 15 min walk to TBIT from T8 if you power-walk. I prefer walking on the lower curb (arrivals). The departures curb upstairs has too many lost passengers being dropped off, taking good bye photos, or saying good bye's. Oh, and there's lines for the curbside baggage check.

However, if given the option I'd go for at least a 2 to 2.5 hours transit time from UA/UX to TBIT. Gives you time to enjoy the lounges!