A lot of time
If this is true then you are already ahead of most cats who wish they could multi-instrument. I've also had a lot of time to shed, for several years now. I always knew I wanted to play another instrument, I figured that keyboard would be the least tedious to cross-over from drums. I did spend several years choosing not to get a keyboard until I was properly "arrived" at my drumming technique.
Eventually I got a little Juno DS and started practicing scales and writing bass lines for my jazz, funk, and hiphop grooves. The keyboard pretty quickly worked itself into my practice routine. It really helps if you are already solid enough at drumming to know what style(s) you want to play on your other instrument(s). For me the synthesizer has been great because I can record walking bass lines for my already jazz drumming. I can write heavy rhythm guitar synthesizer riffs to play heavy metal beats to. I still practice the piano scales, it's important to me to have strong technique. But I'm not even worried about complex melody/theory/song writing ATM, as most of my keyboard time is being devoted to supplementing rhythm section stuff I'm already doing on the drums.
I have thought about getting an electric guitar, but only recently, since getting into djent and nu-numetal. I would prolly get a seven or eight string and only play chuggy, weird djent stuff. I really do enjoy writing funky metal rhythm guitar riffs, my synthesizer has a couple very decent modern metal sounding distortion guitars. But for more complex guitar stuff I might end up doing it with a DAW program instead of an actual guitar. Some of my favorite djent drummers write their own DAW guitar tracks and the mix is
fire.
In addition to the keyboard playing, about 2 1/2 years ago I started practicing singing. I never really intended on learning how to sing, it was like a part of my soul that really wanted to express clean melodies took over and made me get to work. My synthesizer helped me figure out my range and to work on being accurate, pulling notes 'out of the air'. The singing has taken on a life of its own and I'm really enjoying the process. I never know when I'm going to do my singing practice. I just know it's kind of like the way I exercise. It's not on a schedule but I know instinctively I have to do it rigorously, at least once every two days or there will likely be no gains.
Interestingly the most important thing I did to make my keyboard practice way less tedious was just to raise my keyboard stand up so I could walk up to the keyboard to play. It was such a simple thing but a huge game changer. I initially had the keyboard set up all precious with a little bench seat and a music stand. I barely touched it when I had to go sit down in a nook to play, and then scoot out every time I wanted to walk to my V-kit. Having the keyboard up at waist height means I can walk up, practice, record, walk away, repeat. Without all the extra steps and emotional stress of being obliged to "go sit down" to practice.
