SSA to AGPA Timeline?

I got through my interviews and was given a conditional offer for an AGPA position. However, after HR reviewed my work history, they determined I did not meet the minimum qualifications for the position. I was then offered an SSA position. I’m still eager to take the SSA position, but I can’t help but be disappointed due to the potential pay difference. I will most likely take the offer, but does anyone have any idea on how long it may take to be promoted to an AGPA? And what are ways to increase my likelihood of a promotion during my time as an SSA? 

Also, I’m supposed to take the SSA exam sometime soon. I feel pretty confident after reviewing other threads and doing well on the sample questions, but if there’s any advice, I’d gladly appreciate it. Thanks!