I do some of my PCs on HeroForge these days; thus, I'm printing out a new sheet each time the PC levels, at a minimum (more often if equipment changes between levels).
For the PCs that I don't do on HeroForge, I typically stick with one sheet (erasing / revising data as it changes) until the sheet looks awful -- that can take years.
I've got one PC that I've been playing for 26 years, through 5 different editions (Basic D&D, 1E, 2E, 3E, 3.5); I've kept all of her charcter sheets thruogh all the editions -- it's quite the stack now.