Initially they differ from person to person, but like everything else, through practise, you can make them as vivid as dreams. A good visualisation exercise is looking at a picture, than close your eyes and try to recreate the picture in your imagination with all it's details... using all of the 5 senses and imagining the picture to be huge, 3D and bright improves your visualisation skill. It also improves dream vividness and allows easier WILDing. This requires daily practise.. I do it each night before going to sleep.