I say kudos to you for letting your ex spend the time with the kids and that you think he is a good dad. I agree with the other replies. Use this time as time for yourself. Do something you enjoy doing that you don't get time to do when the kids are home. And be thankful they have a dad who wants to be with them.
For what it's worth, I've been on all sides of the "fence". As the kid of divorced parents, as the mom and as the stepmom. It isn't easy either way around. All I can say is don't take the time away from their dad as long as he is a good dad and wants the time with them. I also say way to go for him!
My ex didn't want anything to do with my son, (he left when my son was 3 mos old) and that was pretty much it.
Also as a kid of divorced parents, my mom made things MISERABLE for my dad, so he didn't come around as much as he or I would have liked. It was hard. I loved spending time with my dad.
AND as a stepparent, we have my stepkids every Wednesday for 2 hours and every other weekend (only about 8 days a month). I say only because my hubby would LOVE to have them more but his ex won't let him and it kills him, but he calls them everyday that they aren't with us and he is only allowed to talk to each one for only 10 minutes (but when she calls our house she can talk to them as long as she feels).
I'd say you guys are doing good and the kids are getting pretty balanced time with both of you and that's the ideal situation with parents who are no longer together. And it is nice to see when it happens! Good for you!
Find a hobby to do for when they are gone, read, relax, spend time with friends, watch a movie, go to a movie or whatever it is that you like to do and just change the way you think of the time they are gone and enjoy it.
Just MHO - K.