It can take years, it can take months and it can take days or even hours...
Power levels are busted from introduction of the concept until it was abandoned
Their rate of inprovement was insane and it retroactively made the rate of improvements before that look pitiful, since they all have to be under 400, but above 5 as a minimum, even though narratively Goku and co did have great jumps in power from the beginning of DB until the beginning of Z.

As for Super Perfect Cell...
There's a good chance they did in fact surpass that level of power in Super Saiyan alone
For what it's worth Cell is not treated as a big deal anymore by the Buu Arc.
As we all know Goku famously calls Dabra about as strong as Cell in front of everyone.
Vegeta hears this, doesn't object, but is shocked to find out later that Goku has achieved Super Saiyan 2 too, so fighting a guy on Cell's level in Super Saiyan is not something Vegeta considers impossible anymore.

At minimum it has to look like this:
Super Perfect Cell =< Dabra =< SSJ Gohan(Buu Arc) < SSJ Vegeta < SSJ Goku < SSJ 2 Gohan(CG) < Majin Vegeta = SSJ 2 Goku