Really dependent on your local environment and customer's habits, holistically.
For my customers, I am usually noting 6- 8 weeks for durability for 151, which is pretty darn good for an AIO.
Wiping the vehicle down with D156 after can possibly add another 2 weeks.
HD Speed is a better product to work with in finishing, work time, wipe off, and *the new version* cuts as hard as D151... BUT Speed doesn't last in my climate. 4 weeks tops. Wipe down with HD Express and I am looking at 8- 10 weeks of protection.
D151 is cheaper than Speed and D156 is easier to work with (and faster) than Express.
Trying to present Meg's vs. HD AIOs fairly.
DG501 is fine but durability isn't as spectacular as some state. Virtually no cut. More suitable to light base colors due to the product's great chemical cleaning. You will use quite a bit of product vs. 151/ Speed/ M66.
M66 is very oily with a very mild cut... not even close to 151 or Speed. 4- 5 weeks protection noted, for me. Very cost effective product. Chemical cleaners seem weeker than 151/ 501/ Speed.