How long is the training period?

2 weeks if you've driven before, 7 weeks if you're a new driver. If you don't get it in 7 weeks, they give you another 2, and if you STILL don't get it by then,...

Marty's answer is correct. I will just add that if you have a long gap in driving then you will need to do at least a 2 week refresher course and then you will do 7 weeks training as if you are a new driver.

I have four months experience solo flatbed and the recruiter told me seven weeks.i don't know why

Depends on how much experience you have