This morning when I took my partner to the shops I thought it was a little cold in the Up! so I switched on both seat heaters just as a sort of test run. What an asset they are going to be when it gets really cold. We were delighted with them.
I don't think you'd leave it on for several hours as, after 15 to 20 minutes, the air con (if installed) would be able to keep the atmosphere inside the car at a reasonable temperature, so the seat heaters could be turned off.
Mine turns on at 2 lights until I feel comfortable and then turn it down to 1 light. Then turn it off. I think it is intended only as an initial seat warming device and not to be used all the time. Short journeys may be o.k. but on a long trip I don't think you could keep it on without being uncomfortable.