Well 20 foot is ok if your out running a long trench and have lots of room to move around in and dont care about it being real straight or level but its a hassle sometimes if you run more than a few !
On large projects space storage and long time periods they get warped out of shape ends get bent and you need forks to move it around plus clearing out the pipe with dirt or sand is a bigg issue you cant just pick it up and shake it out .
We kinda like 10 foot in duct banks easyer to work with and in the hot sun it kinda stays level you do more work with 20 foot lengths banging it together its harder to work with but have run a lot of it over the years .
We only use 20 foot today on 4 & 6 inch pvc nothing smaller its a waste of time more labor than its cost and what most think . Sorry but thats my thoughts hope you understand we kinda run a few underground duct banks now and then and have been there once or twice .