Finding places to dump tanks

Sanidumps and RVParky websites/apps.

Impossible to tell someone else how long their tanks will last. Depends on tank size, how many people are using them, how many showers, how many dishes you wash, how many times a day you pee and poop, how conservative you can be with water, etc. etc. Sometimes your tank level lights do not work. You will just have to learn by experience. As an example, we have a motorhome and two of us can go about 7-10 days for the blank tank and 4-5 days for the gray tank. In our case, we can look directly down the toilet to see how full the black tank is. For our gray tank, it first overflows into the shower. You need to learn where that low point is in your RV. But our tanks may be bigger than yours.

Perhaps your friend doesn't need to know you're dumping gray water on the ground.

Pilot and Flying J can oftentimes have dumps -- for a fee.

You will know when it's overdue to be emptied. Flushing will bring the sewer back up and into the floor, lol.

Joke, most RV has light indicators. The rule of thumb is to empty it at 3/4 full. In my RV, that's 3 out of 4 lights are showing red.

There are a few websites/apps dedicated to finding dump stations. Rather than me linking them, just Google for "finding RV dump stations". I've used the Sanidump one quite a bit.

As for how long you can go without dumping, I would first need to know how long is a piece of rope?