When should I transplant autoflower seedlings first growth outdoors out of solo cup.

in my opinion, yo should let them germinate in the final pot, meaning never having to change them. Thi avoid tres for the plant which already has a very hort lifecycle. Yo jst leave a hole for two hand of seedling oil in the middle of your container to let the seed germinate and fill the rest with the soil you normally use.

No different than any other plant. When you can safely pull it out and gently place into a large pot, it's good to go. Enough roots and a little moisture will prevent the rootball from falling apart during the process. If concerned, snip the cup off and it's even less concern pulling it out. Either way, loosen it a bit before pulling it out. 14+ days above ground, i'd wager. I used to use solo cups, but use smaller 2.5" pots now.. that takes 10-12days before they are ready for a bigger pot. A great way to recognize enough root growth to warrant a larger pot is the time it takes between irrigations. If you need to water it daily in that solo cup, it's more than ready... maybe every 2 days.. as long as you do a good wet-dry cycle, that timing is sufficient. You may want an intermediate step or two... 19gal is very likely too large for an autoflower even outside. The sun is a different animal, but it's still a vege-phase limited autoflower, so bigger pot after ~7 gallons probably isn't going to make a huge difference in autoflower growth outside... 19 gallons will remain too wet for long periods of time and likely cause rootzone issues. I used some 7-gal pots outside testing some autoflowers. My backyard doesn't get enough light. They grow aboout 1M tall and were a bit lanky due to lower-end DLI. Even with more light per day, the 7gallon pots would have been plenty. Maine isn't a high dli region, either.. .but if you don't have trees or other obstacles blocking many hours of light, should be fine. 5-gallong might be enough... i doubt they grow more than 5' tall. 1' per gallon is a good generalization for sufficient volume. Anyway, going from a solo cup to a 19gallon size is a bit crazy... it'll be really hard to water properly. The roots formed in a solo cup will never be ready for such a large pot. You'll have to do some weird watering practices to compensate for that context that will more than likely hinder the growth one way or another.