How many square metres?
For example, if its 2000 square metres, it will take between 1 hour and 1 hour 30 minutes.
If its over 3000 square feet then its up to 2 hours.
If its say a large practice of 5000 - 6000 then it will be around 2.5 - 3 hours depending on how many staff you have on site to clean.
There are several factors that need consideration. Floor type. Type of clean. How many floors, density...etc With it being a dental surgery you will find it will be fairly clean. However I would clearly state that you are Estimating ad not quoting and its only a ball park as you need to see the premises.
Also don't forget about TUPE...
We quote all the time by square feet. Very easy to work out.
Hope it helps
Dave