RV holding tanks for sewage will need to be emptied when they are about two-thirds full.
For 2 people using the RV you may only have to empty it once per week or once per month depending on how often the RV is used.
If you have more than 2 people using the RV toilet and RV shower etc then you may need to empty the RV holding tank every 2 to 3 days.
It all depends on the usage of the RV as to when you need to empty them but as long as you empty the RV holding sewage tank when it's about two thirds full then you'll be okay.