The spacing interval is largely dictated by two things:-
1. The depth and thickness of the joist.
2. The span between the bearings at either end.
As the span increases for a given joist depth the spacing interval will reduce, but as a general rule in domestic properties you will space joists at 450mm centres. There's not the space here for a full list of spans and depths, and in any case there's no need.
Fluorescent light fittings are not usually very heavy, and lots of them are fixed straight to the ceiling with cavity fixings, rather than bothering with finding joists. In any case, if a casing is to be fitted parallel to the joists it may not be possible or convenient.