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With most varnishes you want to wait at least 6 months if not longer (if you paint thickly). However I highly, highly reccomend Gamblin Gamvar Gloss varnish. It's a final varnish not a retouch. However like retouch varnish you can put it on when your painting is touch dry. It's what I use and I love it!
Gamvar may be brush applied when the painting is dry to the touch and firm in its thickest areas. For some oil paintings, that may be two weeks, for others, 2 months. To check if it’s dry, gently press your nail into the thickest part of your painting."
So it's safe to apply as long as touch dry and there is no softness to the paint...which as it mentions you can test with your fingernail. They reiterate this in their product video around the 1:30 mark.
Also, a very well respected landscape painter named Mitchell Albala discusses this here: https://blog.mitchalbala.com/gamvar-an-easy-to-use-varnishing-solution-for-oil-painters/
And another painter that talked to their customer service to confirm received this answer to the question of when can I varnish?