Trying to work out a rough time estimate for an ASAT rocket to reach GEO height?
To just intercept the orbit, but not remain there, you can travel at any speed above that required for the Hohmann transfer.
In other words your travel time can be reduced to as short as you wish, provided you have the fuel and acceleration available.
If you can't handle the maths, you could try some trial-and-error simulations with Orbiter for example (just look at the time needed to rise to the altitude you want).
Out of interest (not really an answer), there is a calculator though it only gives the fuel and delta-vee requirements for the Hohmann transfer orbit, not the transfer time. Also it does not do any calculations for the higher energy case you require.