Probably an action but it depends on the effect.
In YS280 it says
Potions are very similar to enchanted items
in terms of function (and, in fact, even use
enchanted item slots to make), but are both
more limited and more flexible.
Since as you said it never describes how to use potions differently than enchanted items they must function like enchanted items do when it comes to using them. So then we look at the rules for enchanted items (YS279) it describes them as:
Enchanted items are intended to hold a single,
pre-generated effect that is stored until released,
and further on the same page describes the what the pre-generated effect can be as:
Nearly any effect within the range of thaumaturgy
and is limited by 2 things:
the effect has a
strength equal to your Lore, and it may only be
used once per game session
This means to me that you are just creating a limited spell effect and would be subject to using your action when using them.
But if we read on (YS279 + YS280) it describes when and what you might have to do when using an enchanted item:
It’s possible that using an enchanted item will
require some kind of skill roll, particularly if it
needs to be targeted in some way; discuss this with the GM and follow whatever course seems logical. Defensive items (ones that provide armor
or a block, for example) often consume a use at
the time of defense and don’t require a separate
action to activate
That to me supports that normally it does require an action to use but depending on the effect being generated you and GM might come up with a way for it to come into play at different times.
As an example, I allowed one of my players to use a potion he created to survive in a "hell like" environment as part of a defensive roll against a beam of scorching sunlight. Again, it's really up to the GM, but I liked the visual of him drinking this potion to stay alive while this demigod scorch a circle around him. Faster acting evocations might be harder to justify but if any system lives on the "Rule of Cool" DFRPG/Fate does.
This answer focused a lot on how enchanted items work but when I've run DFRPG that's how I treat potions, basically enchanted items that you have to create each session.
I hope that helps.