I like schedules, because I think they actually make everything a hell of a lot easier - not just practicality-wise; but in terms of avoiding the battles surrounding issues of neglect and so on. My GF, on the other hand, hates schedules.
What I don't personally find necessary is equality in terms of time spent. What I think works better is whatever suits the current dynamic. Most importantly, I think that anything can work, so long as all the *agreed* responsibilities are being met well.
In our household, my GF, myself and her husband live together, with our four year old daughter. There are no strict rules, but per week we aim to allow: one slot for a secondary, one slot for me and GF, one slot for GF and her husband, one slot for family time. Realistically, we are so busy that this tends to become one official slot per fortnight, with little moments of quality time slipped in every day.
What I wouldn't find fair, for example, is if I decided to make the commitment of living with my GF and family, but decided to piss off three nights a week, because I choose to have three external partners. If I want three partners, I acknowledge that I have to see them on rotation; otherwise I end up neglecting things at home. It goes from "I agree to move in, be there, and be present in all of the wonderful things that come with co-habiting" to "I'm living with you, but I'm only really present half the time. That cool?"
That being said, it all depends on how much of a time suck each situation is. My GF finished with her last secondary because she just couldn't make the time for him that he required. When they physically saw each other, their dates would last about 10 hours including travel time. Then she'd have Top Drop the next day. She didn't see him often, because essentially, she'd end up wiping out two days every week because of him. Then, she'd spend every day texting with him and every night, he wanted to talk online or on the phone for hours. In contrast, she's had much more casual play partners, where the dates only lasted three hours each and the communication maintenance was easier. In that case, it wasn't too much of a problem if she wanted to nip out and see both guys in one week, because it didn't cause her to neglect other things. Do you see what I mean?
It also depends on whether each relationship actually impacts anything else. For example, I was involved in an online/LDR for the past seven months. I used to spend about 2-4 hours a day talking to her. However, I arranged to do this when my GF and daughter were busy with other things, or asleep; so it didn't take away from anything. If I'd decided to jabber away to her on Skype every night through dinner, that wouldn't be very fair. So.. I'd spend what? 14-28 hours a week with my secondary, online. I don't think that means I need to spend 28 hours a week talking to my GF or playing with my daughter. Those hours were my free-time hours - times when I'd just be looking on YouTube, or waiting for work emails, or bored because the household were out or sleeping. Now, if my GF had pointed out that she, or our daughter, were feeling unloved... or if my sex life with GF was going through a shit patch... or if I was in danger of losing my job... or wasn't pulling my weight with the housework... if something was being *neglected* in my life, I'd absolutely cut my time down with my secondary and spend extra time making a new toy for my daughter, or a gift for my GF, or writing her a hot piece of erotica to improve our sex life, or doing the washing, or getting on with work. So, it's more about the balance and making sure everything is healthy.
So, in your case... it will be trial and error, I'm sure. If your BF currently sees his other girls once a week each and still wants to do that, then I think it's fair to schedule one time a week with you, too. Since you'll be living together, regular, smaller shots of quality time (mornings in bed, cuddles on the sofa, etc) should be easier.
If after a month or two, you find that he isn't really as romantically 'present' in the household as he is with his other involvements, you can talk about changing the arrangement. Basically, I think it's less about 'official time balance' and more about ensuring that everyone feels that they are getting what they need and meeting the agreements they have made.