how many hours does it take to chnage out a timing belt on a 91 Lexus Ls400

Get a Chilton manual or factory page at lexus.com for a nominal charge. It's not horribly difficult, but it is very involved. You'll need to remove your fan, water pump, shroud, etc. Being that it's an interference engine, you'll want to make sure that you have your marking EXACTLY correct on each bank. Being that it'll be your first time, you'll spend the better part of a day completing the task. BTW - you'll want to change the water pump as well. Being a former Toyota service manager, I've seen my star technicians change them in as little 45 minutes, but that's rare.

You reply is completely wrong in a very big way. The 1991 lexus ls400 had only one engine offered in it and it is a NON-INTERFERANCE engine. In a unique situation I had myharmonic balancer come apart at 50 mph. The effect is the same as the timing belt going out. Engine turns over but timing belt dose not move. Engine was fine.read the ownrrs manual!