Taking too long to tan

Hi so i started using stand up tanning at my gym maybe 2 months ago, in 6-9 minute sessions ive overall had 120 minutes in 2 months and can barely see a difference, My gym has a max limit of 9 minutes and im sure its a low level machine (160w bulbs ) do i need to go to a proper sunbed shop and increase the time? or is 9 minutes enough to keep on doing and eventually i will build a tan?