From what I have seen, there's no telling how long they last.
Some have the abrasive precipitate out very fast; some are almost gel-like and you never have to stir them up. The ones that do precipitate out can almost always be remixed; I often use a very long screwdriver to get into the almost solid mass at the bottom.Some can easily be shaken up, while others are a royal pain in the butt and have to be shaken an annoyingly long time to get the abrasive back into solution.
Some that seem to keep the abrasive in solution well and rarely need to be shaken are Storm Xtra Shine, Powerhouse Factory Finish, Lane Masters Factory Finish polish & Motiv Power Gel Polish.