I feel this always depends on your style and what you want out of the scene.
If you do a quick smut centered rp, with the scene basically being just your material for a real life masturbation session then you likely only want to extend the scene for as long as you can/want to hold it.
For me personally it's more a narrative experience. Generally, I have a much slower but more detailed writing style, oftentimes just answering once every, or even every other day. While narrative and story is in the center for me there are obviously smutty scenes too, and those can very much last several days.
Yes, just describing pure dick-in-puss action will get repetitive quickly, but I feel for most that's not what they enjoy reading the most anyway. You can write countless iterations of hands passionately exploring a body, of kisses and steamy dialog, f-lists can get you great ideas here too. A gently bite, a smack on the butt, some restraints or toys. In my experience people appreciate it when you go into detail, describe thins powerful and go way beyond just cock and the orifice of your choice.