LST means "Low stress training"

Here... Let me google that for you

It's a matter of preference and growing style that dictates how long you should veg for. The longer you veg, the bigger the plant. But, you need the room and big enough pots and such. If you prune your plants to make them more bushy, you will have to veg them longer to let the new growth develop.

I think the best time to put your plants into flower when growing from seed is when the preflowers start to develop and they start to show sex on their own....usually about 6 weeks or so..but again, it depends on the strain and other factors. Some also say they are ready to go into flower when the new nodes start to alternate sides.