Well your MAXIMUM allowable is the redline area on your tachometer. If you mean by your question the most efficient speed to shift, that should be between 2000 rpm and perhaps 4000 rpm. If you drop under 2000 rpm, you are tending to "lug" the engine, and if you constantly rev over 4000 rpm, you are probably just wasting gas and are in too much of a hurry.
Of course, we all like to have fun, so sure, you can take any gear up to redline if you wish, but generally on modern cars the "sweet spot" or "power band" is designed for the middle range on the tachometer.
If you exceed the redline area, the car will shut down to prevent engine damage.
Your tachometer is a very useful tool if you learn how to use it.
By the way, 5th and 6th gear on your car are "overdrive" gears and generally are not used for city driving unless you can stay above that 2000 rpm range. If you are going too slowly for 5th or 6th gear, the car will protest, seem powerless, and perhaps even buck and hesitate. This is "lugging" and is very bad for an engine.