If you're two-boxing you really should be running a tank/healer combo. You need to get practice in tanking and healing. Once that is sorted out adding 3 dps will be a no brainer afterwards because all you have to do is hit one key for them.

The hard part for me was healing while running instances, took a long time to figure out the best setup and binds for my healer, many iterations.

I didn't jump right into 5boxing though. Back in vanilla I two-boxed first, with no special software. Then eventually 3box, and once I discovered keyclone shortly after I did 4boxing. So I had quite a few stepping stones along the way.

I'd say it took several months before it felt second nature. As in, I could set up any group I wanted and jump right in without making stupid mistakes or being dumbfounded.