How long should Power Cleaner soak in?

Ultramar Power Cleaner sometimes has to soak in for a few hours. We clean a tent or boat cover as follows:

First we clean the tent canvas with lukewarm water and a soft brush.
Then we spray Sprayhood & Tent Shampoo on the tent canvas and let it soak in for half an hour.
Then we rinse away all the grime while brushing with running water.
Then we let the tent canvas dry.
Now you can see if the tent is clean enough. Can you still see stains and grime?
Then spray the canvas with Power Cleaner.
Brush in and allow it to soak in long enough until all the stains have disappeared.
With some tents and boat covers this can take hours.
By spraying it a little more every hour, the effect will be even better.
Only when all stains are gone, rinse the tent canvas with water.