All varnishes set in two phases. First the solvent evaporates as it dries, then the as the resin molecules come into contact they cross link and cure. The only concerns with the actual curing time, which can be up to month, are that you follow the minimum time between coats on the label (generally about a day) and preferable wait the month before eating off the finish (which is hardly ever an issue).
As far as the sun goes you only need to worry about the dry time, i.e. how long do I have before enough solvent evaporates that my varnish is the consistency of peanut butter? The answer is a combination of the product in question and experience.
Traditional varnishes are very thick to begin with, and the instructions call for heavy thinning of the first coats and progressively less thinning of additional coats. They also require sanding every coat so that the layers will bond together. The need to measure and mix chemicals and laborious application does create a marketing opportunity for simpler to apply products.
If your varnish doesn't have a thinner schedule in the instructions (Minwax Helsman or similar) then it's thinned at the factory to an average value and contains additional chemicals to allow it to "burn in" to undercoats without sanding. There are tradeoffs which aren't relevant to the question at hand. What is relevant is that you'll find lots of different directions depending on the product.
Your best bet is to work with small amounts in a separate container. That prevents you from contaminating the varnish with dirt and allows you to keep it tightly covered out of the sun preventing the solvent from evaporating. The amount you can keep in your working container depends on the conditions and how quick you are applying it. If it's graduating from paint to frosting you're using too much.
If the label indicates "Clean up: Mineral Spirits" you can also thin the varnish with them if it begins to dry out on you. You can also thin it slightly from the get go if you know evaporation is going to be an issue.