I'm sure that your daughter already senses that something important (and potentially bad) is going on in your home. The timing of speaking to her is the least of your issues.
Young children in stressful situations worry about three things: Am I safe? Are the people who take care of me safe? How will this affect my daily life? They also tend to blame themselves for anything bad that happens in their family. You need to address all of those issues.
First, hug your daughter and tell her that you will love her and take care of her forever and ever. You should do this a lot over the next year. Remember that the details of your breakup and custody arrangement are beyond her comprehension. What she really wants to know is that she'll be protected and that you're OK.
Reassure her that she's a good girl, and that she didn't do anything bad to cause this. Again, it's a good idea to say this many times.
I strongly urge you and your boyfriend to get some counseling from a child psychologist or child psychiatrist who has expertise in divorce and custody issues. A local family court judge can probably help you find someone.
While a 50/50 split may sound good to parents, it may not be the best thing for your daughter, especially since shuttling between two homes every week or so can be extremely stressful to a young child. You should explore other possible arrangements, with the understanding that these may change as she grows older.