1990 Geo Prizm Head Gasket and timing belt

In 1990 this job would be about 6 hours but with rusted busted and seized bolts you could expect it to be 8 to10 hours

To add to what mastertechtim said:

You should replace the waterpump while you have it down.
Make SURE you keep the lifters and shims in the correct order, they MUST be reinstalled on the same valve they came out of.
If you have blue smoke at startup, now would be the time to change the valve stem seals, but you need a special service kit to do this, Toyota may have one.
Be very carefull when removing vacuum lines on an engine this old, they can break very easily.
I would also replace the front crank seal while your at it, it is in the oil pump housing.
You should replace the head bolts if replacing the head gasket and check the head for warpage. Re-tap or chase the head bolt threads in the block, and make sure the exhaust bolts/studs are replaced and the threads are not stripped.