There’s no set time limit on our vacuum mounts. It depends in part on the surface you attach to, the condition of the vacuum mount, the load you put on it, and other factors. As for surfaces to attach to, vacuum mounts in general were designed to carry big sheets of glass and other solid surface materials, so smoother, flatter surfaces are ideal. Clean surfaces and clean SeaSuckers go a long way to maximizing the hold time. In any event, if the vacuum mount begins to lose some vacuum, it will warn you so you can get it back to full power before it comes off. As you probably already know, the vacuum pump’s plunger will reverse itself and extend from the pump cylinder when it starts to lose power. Once you see the orange indicator band, that’s your cue to repump your SeaSucker. For car racks, the general rule of thumb is to check the rack before you get in your vehicle, and check it each time you stop for gas, food or the bathroom.