I don't think there is a single correct answer to this question in B&W or colour.
Originally Posted by FilmIs4Ever
The obvious answer is to follow the manufacturers recommendation, which will normally state so many ml replenisher for each film or sheet of 10x8 paper. Replenisher itself may be a normal strength working solution or something much weaker, whatever it is will be disclosed in the data sheet from the manufacturer.
I tend to take note of the manufacturer recommendations and then once a solution is supposed to be getting exhausted watch carefully for signs of exhaustion. If I see an indication of exhaustion, I replenish :-)
I also look for missing solution, if I mixed 600ml originally (which would fill a bottle to the top, and now I'm down to 500 ml, I'll mix another 100ml of working strength solution and add it to the old so thatI continue to deprive the solution oxygen when I bottle it.
I do use a propane/butane squirt (cigarette lighter fluid) in every bottle before I close it to remove the oxygen, which helps a lot with longevity of developers.
Some metrics for you. I'm still using patterson RA4 dev originally mixed in November 2004, replenished 3 times to make up volume. The Patterson Bleach-Fix has been replenished once to make up volume. I have not once replenished because the chemistry is showing signs of exhaustion. I use this chemistry at room temperature (~24C)
I use Agfa C41 colour kits typically for 8 x 120 films rather than the 5 x 120 that they state. The kits easily last 4 months from mixing as long as they are stored back in airtight bottles after each roll of film.