My impression is that they don't last as long, but definitely for long enough. People think that if you need to heat them more often then it's less efficient,

My impression is that they don't last as long, but definitely for long enough. People think that if you need to heat them more often then it's less efficient, but if the heat doesn't last as long then almost certainly they take less energy to heat up in the first place. It's all about transferring energy/heat from one place to another.