There is no way to set the number of "ticks" for the "dissolve" effect. The faster your machine, the faster the dissolve functions. The other transitions in Media Tool do let you change the duration, but not dissolve.
On Windows platforms, the dissolve feature may often appear blocky or not show a transition at all. If your video card supports it, try setting the colors to thousands, or 16-bit color. This allows more colors and creates smoother transitions.