If you must do something like this, perhaps the average number of landings needed would be more appropriate than the time taken? Some people have to do orbits, back-tracks, holds, extended circuits which all add time.
If you are asking because you're worried because of the amount of time you are taking - don't be. It might be because of the reasons above and if you so you are learning other skills rather than landing (even it it's just observation as you wait for someone else to take-off!). It might be because the weather is causing big gaps between lessons and holding you back - don't worry, flying is like that.