If you cannot increase the temperature, you will have to modify your formulation. Here it is important to know whether the paint dries through solvent evaporation of through chemical reaction. If the latter, there will usually be more possibilities of achieving your aim.
For a solvent-based, clear lacquer, a drying time of maximum 30 minutes for the film to become touch dry and 24 hours to become dust dry is a good guide value, which can easily be achieved. Complete, hard-drying should take no longer than 24 hours.
Drying times for pigmented finsihes which dry through chemical reaction should be rather less. Different drying times will apply to primers and metallic finishes (if these, in fact are made in air-drying formulations). Generally speaking it is advisable to modify the type of drier and be amount used. The choice is a wider ore and metal salts are commonly used.