using pk 13/14 ??

i have used pk 13/14 before but have recently been told by a friend that i can use it for a longer period of time than the bottle suggests, the bottle tells me to use it once 3 weeks before harvesting for 3-6 days if i use it for a longer period of time will it harm my plants as i thought it was only contained phosphorus and potassium which i thought the plants want most during flowering and would only benefit them ????

I use Canna nutes in a recirculating aero type system with a 10 gallon reservoir. On my last grow I would change my nutes out about every 2 weeks. When I used the PK 13/14 I left it in for the 2 weeks until my next nute change. I never had any problems. None that I knew of anyway. I would also top off with water when needed in those two weeks.

Id like to here someone that has more experience chime in on this question