About 1.5-2 hours, if you aren't changing the timing belt at the same time. If you're very good and take lots of shortcuts you may be able to get it done in an hour, but your first time out might take you as long as 3-4 hours. Be sure to have the Toyota Supra Repair Manual (Engine) on hand.
- Remove covers
- Remove tensioner
- Mark belt with chalk or white marker at timing marks
- Slip belt off
- Remove cam gear bolts- Remove old gears
- Put new gears on (properly-torqueing the center bolts)
- Slip belt back on (ensure timing marks line up perfectly)
- Reinstall tensioner (Don't over-torque these bolts!!!!)
- Recheck timing marks
- Set -4.5/+1
- Reinstall covers
The best way to hold the cam gears steady while either removing or retorqueing (other than removing the cam covers and using a 32mm wrench on the cam itself) is to wrap an old timing belt around the gear and then use a chain vice grip - be sure to double-up the old timing belt where the handle s on the chain vice grip puts pressure on the cam gear (depending on the way that you're turning the bolt in the center).
Be very careful when reinstalling the tensioner - tighten the two bolts holding it in a bit at a time on each side (2 sockets with 2 extensions are required since when you tighten the back one, the tensioner tilts towards the back of the car and you can't easily get the socket and extension out). Do not overtorque these bolts (I've had to deal with a case where someone towed a car to me with both bolts broken off - screw-extracing them was a real b*tch!). Use loctite and torque them exactly according to the Repair Manual.