I've run the Sky at midnight, and so have many others so really... there's no limit.

But if you prefer boating in daylight... Sunset is currently a little after 8PM now. So, it all depends on how long a section you plan on running and how much time that normally takes you (and the group you intend on boating with). If you're doing a short lap from the confluence and taking out at Split Rock you're probably fine putting in at 6:30 or even 7:00 now as long as you got a crew that is comfortable just heading down and not mucking around a lot. If you plan on scouting/portaging the drop or if you go up with a big group figure on more time.

Also keep in mind, every day is longer than the last for the next 2 months so in two weeks, 4 weeks, etc it'll be a little bit different story. By late June you can drop in at 8:00 PM, bomb a lap and still get to the takeout with light.

More info than you needed, maybe?