Have you ever seen that movie star that changed from blonde to brown, and then back to blonde. Oh, it doesn't matter which, you can think of one I'm sure. Did you ever wonder how big of a deal that is? I'll give you a hint...it's huuuge.
The blonde to brown isn't that big a job. For a good colorist you can do it well in around 1 1/2 hours. I say well, because it's easy to turn it to baby poop brown. I've seen it, it ain't pretty. And going brown to blonde isn't a huge challenge. Again, about two hours.
Blonde to red can be tricky, but not usually as time-consuming. A number of glazes can be done in as little as 2 hours, if you know what to use. But red to blonde, totally up in the air. Red is one of the hardest colors to remove, there always seems to be that last little bit that doesn't want to budge. It's ironic considering how quickly red fades!
But blonde to brown to blonde is a whole other world. It can easily take 2 or more visits to remove the artificial brown. There are some simple ways to go to a subtle blonde. In a few visits enough foils can be added to get you there painlessly. But if it's to be a true blonde it usually involves a color remover (1 to 2 hours), a bleach wash (15-30 minutes), a filler (45-60 min), a glaze or two (1-3 hours), and possibly foils (1-4 hours). This is all rough time estimates, length and thickness of the hair, porosity, darkness of brown level, and many other factors can affect a color correction of this sort. And all of these are based on the hair being healthy. Bad "at home" color is another game altogether.
I am rarely willing to tackle a black to (literally whatever you can get), but I gave it a shot for a dark blonde client who had "accidentally" tinted her hair black for Halloween. I say accidentally because she didn't read the fine print on the box of semi-permanent hair color she used. Hint: no such thing as semi-permanent black. Once you go black, you never go back, as we say in the hair industry. What, you thought that meant something else? Your bad...
Anyhow, after the first day (5 hours to get her from black to dark brown, ugh! I don't like to fail), I went home, tinted a doll head black (using the same color product she used) and proceeded to spend the next two days getting it blonde. It only took me 3 hours to do it on the person. It was amazing what I learned!
On the other hand, I accidentally tinted a blonde client black myself one afternoon (it's really not my fault. The blonde (6•1) looked exactly like the black (1•9) when held upside down!). And it only took 5 minutes to fix that! luckily I know shortcuts.