Okay I'll try that but thanks so much for the advice!

if It turns out that I can't find any, what's the minimum it could take? Am I right In thinking that 7 weeks is a maximum?
As I stated before there are a lot of variables, some we understand, some we don't even know exists. There are some things you can do to help the bacteria a bit.
The sweet spot for pH is 8.0-8.4. You can add some baking soda (sodium bicarbonate) to help raise the pH.
The temp needs to be around 84F (~30C).
But, warmer water needs extra oxygen because warmer water just can't hold as much oxygen (necessary as the bacteria you are trying to cultivate are aerobic - need oxygen). The easiest way to do this is keep your tank water level a little lower than the return from the filter, so that the water splashes back into the tank. This disrupts the surface, creating more surface area and allows for more gas transfer into the water.
When filling the tank, use your dechlorinator at 1.5x-2.0x the recommendation by the manufacturer. Sometimes the water companies add a little more chlorine, so the extra dose helps. HOWEVER, don't go above the double dose level as the sodium thiosulfate can disrupt the N-bacs.