you can keep an auto in veg almost at will by removing any leaf that's splitting into more than 3 points. beyond that there's little that you would actually want to do like mess with the environment so that the plant stays in veg (cold works!) or less light like 4-6 hours.... autos are autos, try these new gens : fast flowering photoperiods. I did a full cycle in 12 weeks and was very impressed ! for you you could keep those in veg for as long as you want then flip to flower and perhaps be done in 4-6 weeks (which is short !)

If you are asking how long you can veg for in general, then indefinitely if you look after it! In ur situation I'd probably flip now the autos will still do fine they might take a bit longer to finish but you will still get a good harvest. Plus if you take some clones from the photo plants b4 flip you cld have some monsters ready to go in to flower when these are done! ✌️💚👊