Do you know what type of strain your plant is? if so have a look on the planet skunk website and under seeds you can look up how long in general to bud your plants for. as quoted some can take up to ten weeks where as some can take only 6 weeks.
Also this site tells you generally how much yield...
If you tie the laterals down so that all of the heads and growth points are relatively the same height then you can get away with a much bigger plant for a fraction of the height, Thats ho im managing over a pound per plant, takes a bit of time to do but who doesn't enjoy sitting with their...
What I would do is flush the plant with a water solution for as long as it takes for them to start looking healthy, get yourself a ph meter as soon as you can afford i, if possible get one with moisture sensors aswell. This may or may not help your problem but is a good way to start.
I have just purchased a 1200 x 1200 x 2000 cm square grow tent and have the option to buy either a 600 or 400 watt lighting system, Which would be best for me?