A "suborbital" trajectory is just an orbit that intersects Earth's surface, and in theory you can make it whatever size and duration you want. I suppose the limit would be when your apogee becomes high enough to be near or beyond Earth's gravitational sphere of influence, which would happen for orbital periods on the order of months.
Practically speaking, if you have enough thrust and delta-v to achieve a suborbital flight of more than a few tens of minutes, you also have the ability to reach orbit, so why bother?