From the HotS site:
Update: Greatfather Winter has declared that Winter Veil will conclude on January 12, 2016.
A few games a day will go a long way to getting you graves. The stimpacks are awesome for experience. Also, if you're grouped with someone on your friends list, you get an additional 50% experience.
I only play the vs. AI right now, but groups are quick, and games are generally in the 15 minute range. If you prefer to play solo, click the vs. AI, and in the bottom left corner, check the box that offers AI teammates. This game can be paused at any time. I use this mode when I am learning the heroes and honestly, when waiting for pizza delivery.