The general advice is to leave oil paintings at least 6 months before applying varnish. That is because the oils in oil paint cure rather than dry.
I only finished my painting last week and although I didn't use thick undiluted paint, it still won't be ready for normal varnish.
The problem is that parts of my painting appear matte and the colours a little dull without a coat of varnish
I need to have it ready to hand in next week so I will have to apply a couple of coats of spray retouching varnish and hope that does the trick.
Wish me luck! Last year I got third prize for my painting, "The Procession", Let's see what happens with "The Floral Offering" this year.