Hey...welcome and good luck with your endeavors~
The weather, for me, plays a vital part in my work outside, as I know it does for any contractor~especially painters. I have radar on my cell phone, so this helps me to be on top of it quite well, as I am able to stay ahead of rain...
It's one thing to 'prep', and another thing to do a full prep job. Are you sanding the exterior to bare wood? are you priming? Since it has rained a ton this season, I've had a rough time keeping up. Sometimes it would rain all day, and sometimes it would sprinkle. 20% chance means I work outside all day.....60% (and a view of a storm on radar)is a different story.
If you sand to bare wood, make sure and seal it before a drenching rain. If your just priming, you can be a little more flexible. If rain is unavoidable, see if you can do any soffit or underhang work. It also makes a difference on the exposure of the side your working on, the dry time of the primer, etc....in other words~factors could work in your favor, or against.
After a good rain, I usually give a house 1-2 days nice days to dry(if it is well sealed). But a moisture meter is a good thing to have. I find it more of a judgement call when you talk about how much time to paint before it rains. 1 hour is a lot different than 8 hours~lets put it that way.....