My seedlings in ground will take from 4-7 years to fruit. If the tree is a vigorous grower, and it reach above 7 ft then it will be ready to start flowering and usually fruits the first time you see flowers on the tree. But the fruit quality will be best in 2 years (or second season of fruiting).
I do have one seedling tree now that is only 6-7 ft but on the skinny side (1-1/2" diameter trunk), growing in a pot (roots escapted from pot holes), first time fruiting and has four clusters of fruits which are excellent tasting (white flesh) so it’s getting assigned a name and I will be keeping it (KT18-Kim).
I did hear from Mark Lee that Jim has so many loquat seedling trees that it was hard to remember which ones were the good tasting ones that were not so popular (Jazzy, and many others). I try to keep my seedlings ID as soon as it gets taller than 7 ft or 4 yrs so if it is good, I give it a name, if it is average, I use it for rootstock, and it doesn’t get a name.