God loves a cheerful giver. 2 Cor. 9:7; Jehovah's Witnesses can tithe if they choose to do so, but it's not necessary. They don't believe the Bible or God requires tithing. 
They believe in 'voluntary giving' They give out of their abundance, and not a strict percentage of their income. 
No collection plate is passed around and no prayer is said for God's blessing on donations at each service. Instead at the back of each Kingdom Hall, JW's drop money in a donation box. Often it's mounted right on the wall.
People can give as much or as little as they want. All donations are anonymous, unless of course someone puts a check in the box instead of cash. The organization does not keep track of how much each member puts in the collection / donation box.
That being said, when Jehovah's Witnesses die and don't have any family, members may choose to donate their estate to the Watchtower. The Watchtower has historically been a very wealthy corporation but in recent times has fallen upon tougher times.
Many JW's have left the Watchtower Society. JW's are required to shun everyone who leaves and not associate with them even if they are family members.  There are some exception such as minor children or a spouse living in the same house, but other than that there is no contact with 'apostates'. Therefore it is quite common for a JW to die and even though their family is still alive they haven't spoken in decades. In situations like this it is unknown how many JW's leave everything to the Watchtower.
Jehovah's Witnesses attend 5 meetings a week plus small get togethers before going door to door. Some meetings are back to back. In other words one right after the other. Thus a donation box makes more sense and since Jehovah's Witnesses don't believe in tithing, a donation box allows JW's to give voluntarily.
Although it's not called a tithe, JW's are expected to donate their time to 'Jehovah's organization' the Watchtower. Time is money. Once you've spent your time, it's gone for the rest of your life. It's a precious commodity.
Here's what is expected of Jehovah's Witnesses unless they're not physically able to do these things.
Attending all meetings
Preparing for meetings and participating
Preparing to deliver public talks and/or teaching demonstrations
Private Bible study
Reading all new Watchtower literature from cover to cover
Field service. i.e. preaching door to door. This time is reported by each 'publisher' and tracked by the organization. Years ago regular publishers were expected to put in 10 hours per month. It was a goal but it wasn't achieved all the time. Today 'regular publishers' are not given an absolute quota, but are told they "could set goals such as reaching [the] national average of hours for publishers."  
1. Watchtower, Dec 1, 2002, Article: 'Giving That Brings Joy'
2. 2013 Convention of Jehovah's Witnesses. Video: Shun Your Family
3. "Meetings to Help Us Make Disciples", Our Kingdom Ministry, November 1987, page 2, "...set goals such as reaching national average of hours for publishers"
4. Organizational structure of Jehovah's Witnesses. Wikipedia