Span the joists 12” O.C. for tiled area
the spacing of floor joists may require 12" o.c. depending on the span of the floor joists, size of joists, physical properties of the joists and the type of tile being installed. The typical code minimum live load deflection limits is L/360 (L = span in inches). If you were installing marble tile the Marble Institute of America recommends a deflection limit of L/720, which is half of the code minimum. To accomplish the higher limit for deflection would require either taller joists, less spacing between the joists or a combination of both. Limiting the span of joists would also aid.
"You get what you pay for, and sometimes free costs more!"