I recommend doing a web search for "force carbonation chart" and "force carbonation" in general.
Assuming the 7,75 PSI is at 32F and you want around 2.5 volumes of CO2, it takes 5-7 days carbonate.
As side note, I have been working on a formula to help folks calculate the time factor based on PSI and temperature. This seems to be lacking in brewing tools. Folks like me usually suggest a"about a week" because that is what is usually is. As with many brewing calculations, mine looks to be kind of close but can't be precise due to many external variable conditions.
Cheers!