A topper can stay on for as long as you like, while you're staying in one place. If you want to close the roof and drive away, it has to come off.
How easy it is to fit on depends a bit on the type and make.
A whole cover, closed on top, is relatively easy: you open the roof just a tiny bit, you drape the topper on from front to back, and then you open the roof completely, thus raising the topper with it, and you fasten the fastenings (also different for different make of toppers).
An open topper (without roof, for use with solar panels or luggage boxes etc. on the roof) is somewhat more tricky, depending on the make. But basically the same procedure: you open the roof a little, you walk around the roof with the topper, draping it on, close it at the back, and you raise the roof with the topper on and then fasten it.
We find the german topper, where you set little pins in the roof rail (and leave them there) over which you put the ringed holes in the topper's top seam, a little easier than the Comfortz system, where you can't really attach the topper to the sides while you walk around, there is just a velcro fastening at the back - it tends to slide off or is too large a piece of cloth to handle when in high winds, etc.
With a littlebit of practice all very doable!
Do you really need a topper? Is one necessary? No, many do without. If you're just using your Cali in the Italian summer, why would you want one? We are often out in not so good weather, and then we really like it. Especially in winter it is a good isolation, very comfy with the heater on, it keeps the bellows dry and clean, no snow drifts in the sheering mechanism of your roof, etc.