Paint jobs are 99% preparation 1% paint
Any "variation" in time will be due to problems encountered and the difficulty in trim removal and replacement.
As far as I am aware there are no "standard" time sheet/costings for an E9. It was built and serviced commercially long before the accountants, time and motion people entered the fray!
However I would consider the size and levels of trim to be about the same as a modern Jaguar XJ6 so you could use Jaguar costs as a rough, and I mean rough, guide.
In the UK painting a car is not cheap. Even if you strip the car to a rolling shell your looking at the thick end of £1500 (~US$ 3000) just to prep and paint. Any remedial work soon sends the costs soaring.
Depending on the bumper type, either standard or American impact, and assuming that all the nuts and bolts can be freed off easily I would estimate
about 1/2 hour for the front.
The rear will take a little longer, the boot side trim has to be removed, plus the number plate lights wiring has to be disconnected.
As the impact bumpers are heavier I suspect it will be a 2 person job.
Putting them back on will take longer as greater care is required. Remember the socks on the bumper ends to stop the paint being damaged! (Great tip)