The schedule of replacements on both devices is directly related to your vaping habits, juice viscosity & flavoring. An example, I was using a carto in a carto tank (IBTanked) and ran through 50 mls of the liquid and the carto is still performing flawlessly. The juice was a citrus type with no color. Same tank - different juice - heavy, dark and syrupy - 1 full tank full is all I got through before the carto completely clogged up.
There really is no specific time limit, replacement will be determined by taste and draw, when those drop it is time to replace.
The heads on the ProTank can be cleaned and used again and again as long as care is taken. There are several threads concerning the care of ProTank heads etc. throughout the forum.