It depends on the walk. There are some routes on this site, described as one-way walks, but most of them are "allez-retour". It is usually indicated in the description of the walk. An example: this route I did last weekend:
http://www.walkhighlands.co.uk/moray/nairn-cawdor.shtmlThe distance says: 9km, 2 hrs. If you look at the map, it's obvious that these numbers are for one-way walk only. Plus, the description says that "a bus can be taken for the return."
I know that it can all be confusing, sometimes the times given in different sources are overoptimistic and sometimes far too pessimistic
I tend to rely on distance and ascent numbers to calculate my own time. When double-checking with WH times after the walk, I find my time within the range given here, though me and my chum are usually closer to the "faster" end of the spectrum
It all depends how fit you are, how many breaks you take to take photos/rest/admire views, and of course on the lovely Scottish weather
Cheers,
BP