Below are few highlights for Named and Document CALs:
Named CAL :
1. Assigned to a unique and identified user
2. User can access as many Qlikview docs as may reside on server or server cluster.
Document CAL :
1. Assigned to a unique and identified user.
2. User can access only one doc to which Document CAL is assigned.
3. Multiple Document CALs can be assigned to same user.
Based on these, I don;t think there is any restriction for time or concurrent users.
Hope this helps!