Basically all you can do is to help him heal from his surgery and continue to monitor his activity, eating and for any additional lumps.
Its nearly impossible to estimate how long they have left. If the tumor was removed with clean margins you may never see a regrowth of that tumor. If a small piece of it remained, or it had already spread, it will really just depends on the tumor in how fast it grows. Some are prone to grow quickly, others have great blood/nutrition supplies and grow quickly, and yet others grow so slowly that you get a lot of time before they become problematic.
It is an emotional roller coaster dealing with tumors. What I try to do is to consider this extra time. Having the tumor removed hopefully will give us extra quality time with our quilled friends. Enjoy every day you get where they are not suffering from a tumor but monitor and make decisions one day at a time.