I found that it was best for me to sit down and just write a candid answer to each secondary prompt, give it a quick edit for spelling and grammar, and then submit. You shouldn't need to spend more than a few hours on each secondary. My best advice is to devote a weekend day to secondaries, take your laptop to a place where you know you work well (your room, library, coffee shop, etc.) and just bang them out at once. A lot of prompts are similar so you'll end up reusing sentences or paragraphs, and it's easier to do this when you're writing them all at once. This is how I did my secondaries and it was successful for me.
Sometimes I think the amount of time you spend on a secondary means nothing; just write honestly. Sometimes you'll give the type of response a school wants to hear, sometimes you won't. I spent a lot of time on NYU's interviews and ended up with an interview when I probably wouldn't have gotten one otherwise. I also spent a lot of time on Loyola's and Wake's secondaries, only to get rejected from both at the end of the cycle. I hardly spent any time on SLU's secondary, but I wrote about my experiences with the Jesuit tradition that were very important to me--clearly my answer fit with the school's philosophy, because I got an interview invite three days after submitting my secondary. It really just depends.
Oh, and as far as answering "optional" questions--only do it if you have something truly meaningful to say. If you're filling it out just to eradicate blank space, it will be obvious.
