How far apart should timber joists be for an MDF floor?
The standard flooring material is OSB, not MDF.  MDF doesn't have enough strength to support a floor load.  I can't remember if it's 3/4" or 5/8" OSB, so someone will hopefully chime in with that spec.

Typical spacing for joists is 19.2" (just under 3/8ths, there's usually a black diamond on tape measures for this point).  It corresponds to 5 joists per 8' span.

Note that if the joists will lay directly on concrete, you need pressure treated to prevent moisture damage.  And when nailing into pressure treated wood, you need galvanized nails to avoid a chemical reaction that would eat through the nail.