No, there's no scale here, since the number of squares depends directly on the theme. For example, a frequently occuring theme pattern is three 15-letter entries, for a total of 45. Another frequent theme pattern is four 10-letter entries for a total of 40. Both are fine -- but the theme length determines the number of squares.
Below 35 is poor -- for one thing, it would be hard for the solver to identify the theme. But there may be terrific fantastic themes where 35 or below might be acceptable -- better be a darned good puzzle, though. Above +45 is nice -- five 15-letter theme entries, for example. But as well as a good theme, a puzzle needs to have good fill. The longer the theme, the more constrains you have on the fill.