I promise you you wouldn't if you had one. Speaking from bitter experience it would soon start to irritate you if you were regularly paying for 4 hours and the job was only taking two, especially if paintwork wasn't getting wiped down or windows weren't being cleaned, or sofas were being vacuumed around every time, rather than under or behind. Lots of cleaners get complacent and go on automatic pilot, just doing the bits that show and obviously need doing each time, never using the extra time that you are paying for to keep on top of the less obvious jobs too.
When they initially price up, they should be doing it on the basis of how long they think it's going to take them. That's why you should ALWAYS ask for their hourly rate. If they say they don't have one they are lying. Of course they do. Everyone does. Ask them how long they estimate your house will take as a regular maintenance clean, not one-off deep clean. Then when they give you their price, that will give you their hourly rate.
If they say it should take 4 hours or whatever, that is what you will be charging you for, so why would you be happy to see them consistently leaving in 2 or 3 hours? Either it never needed 4 hours in the first place and they over-quoted you, or they are rushing or skipping stuff and not doing it as well as they could be.
When my cleaner first started my house really needed a good deep clean so it took longer for the first couple of sessions. I told her I'd happily pay for as many hours as it took to get the whole house up to scratch over two or three longer sessions and then she could let me know how long she thought about a regular clean after that. We settled on 4 hours a fortnight as a maintenance clean, but she's frequently done in three. When that happens she lets me know and she asks if I'd like something extra done, like some windows done, or a bit of ironing, or the oven or something, or if she prefers to get off early she'll do that. But she only charges for the time she's there. If she's done in 3.5 I still pay her for four because I don't want to be mean over it, she works so hard. But equally there are times when we've been away between cleans and the house has barely been used, so if it only takes her two hours that's all she charges for. We are both happy with this and it works well.
She's fantastic and because she's so fair and transparent and I don't feel she takes the piss, she will get a good Christmas bonus when the time comes. Other cleaners I have had in the past started well but very quickly got lazy and complacent, leaving earlier and earlier each time, even though there was clearly still things they could have found to do with the time they were charging me for. They didn't last long with me. I'm not mean but I won't be taken advantage of.