If you have 3 apples [lumens] and someone gives you 2 more apples [lumens] how many apples do you have?
Of course lumens add... why else would people use more than one light?!?!
As to the main question: It depends on how big you want your plant to get. It will continue to grow as long as it has space, food and light. If you are doing a small personal grow, let it veg about a month to 6 weeks then let it flower til its ready (depending on the strain).
he's right, lumens don't add up.. if you have 4 3000 lumen bulbs, you don't have 12k lumens, you havre 4 3000 lumens.. for instance, you're not going to get the same lumen output form 4 3000 lumens bulbs, which are probably about 23 watts each, as you would from one 250 watt'er which is probably about 25k or so lumens.. you still only got 4 bulbs, each of which are only 4k each.. i know it's a bit confusing..
but as far as how fast is hydro vs soil. there really is no difference between the two.. hydro you get bigger growth faster in veg, but it still takes a 10 week strain 10 weeks to flower regardless of medium.. the plants might be bigger in a hydro setup vs a soil run, but they will still take the same amount of weeks to finish..