first off some of the earliest flowering strains take about 50 days to flower some of the longer flowering sativas could take 100 days or more to flower so how long a plant flowers for is dependent upon the strain or in hybrids or crosses you may have different phenotypes that will finsh flowering at different times so you could even say it depends on the plant the flowering time given by a seed breeder is not always accurate.
how much veg time really comes down to what style of growing you are using in a sog with 3-4 plants per sq ft you may not have any veg time and just start flowering rooted clones.
you can also veg your clones for 3 weeks and use 1 plant per sq ft just give each plant more space the longer you veg, the more space the plant will take up with the right lighting all methods will yield close to the same.
in the case of efficiency in both time and electricity usage sog is probably the most efficient as there is little to no veg time but you will have alot more plants to fill up your grow room.
if you veg clones for really long like 8 weeks to get large plants light penetration comes into play where it may require multiple lights for good coverage and to equal the same yield.