Usually not very long. I empty the reservoir with a syringe, refill with clean, and bleed it through until close to empty, refill and go. If still squishy try cracking the banjo bolt under the reservoir. Put a plastic sheet over the paint with an old towell on top of it to soak up any drippage (brake fluid bad for paint). Then I wrap the banjo (with wrench already on it, 12mm I think) with an old sock, squeeze the handle and just crack it. That will get any air out trapped up high.
Then clean it up. When you put the cap on the reservoir watch for additional fluid that may seep out when tightening the screws.
Front left caliper first, then rear. They operate on the same reservoir under battery. The front right is run from the right handlebar master.
Make sure not to let your reservoirs drain out while bleeding, keep topping up as you go.
Lastly, buy some speed bleeders http://www.speedbleeder.com and your good to go.
try this, I stole it from another thread. Good Luck !
There is a trick for the GL1500 rear brake setup and Bleeding. There are other posts where I have explained this but here you go.
The 1500 front brake cable is very long and does some twists and turns. So you bleed the brakes always do the front first if you don't do the front first you are just wasting your time. After the brakes are bled (by the way to do it right you need one of those one man bleeder bottles from the auto parts store usually for around $10.00) you put the bottle back on the front brake push a little fluid through the bottle then put something heavy on the rear brake pedal (I use a sledge hammer it works great) Leave it overnight you will usually be surprised in the morning to find air bubbles in the fluid. They just seem to work themselves out overnight. Tighten the valve back up and see how your brakes are.
One thing it is always good to bleed the front brake also. Check the color if its not pretty clear bleed them. If the fluid gets dirty it can plug up the return hole. If that happens the brakes will work one time but will never releasethe pressure and hold your wheel locked tight untill you release the pressure which of course means afterwards you have to bleed your brakes anyway so why not save yourself some trouble.