How long should I leave my coffee for in a cafetiere (French press)? I'm struggling to find a good way of timing how long to let my cafetiere stand before drinking.

Is there a rule of thumb for how long I should leave it per mug? Or is there more to it than that?
Start with a coarse ground of coffee with about 1 ounce (28 grams) of coffee for about every 15 ounces (450 ml) of water. The water temperature should be below boiling, about 200-205° F (93-96° C).

Stir in about 1/3 of your water to let the coffee grounds "bloom" for about 30 seconds; then add the remaining water, cover it, and let it steep. The total brew time should be about four minutes. Then press the plunger to the bottom and pour. 

Once the plunger is pressed, be sure to dispense the coffee into another container once that brewing cycle is done. If you let the brewed coffee sit with the spent grounds, it will start to develop a more-bitter taste and be somewhat over-extracted (unless that is an effect you are looking for). 

Note: There is also a "cold-brewing" method which requires several hour of extraction, but that is likely to be outside the scope of what you are asking.