3 months until alkyds are fully cured and can be varnished. Most sources say 6 months for regular oils, but they will both depend on drying conditions and the thickness of paint.
A tip: Dip a lint-free rag in a solvent and rub gently on the painting surface. If colour shows on the rag then additional drying time is needed. If not, your painting is ready to be varnished.