With a continuously running clock do one pull-up the first minute, two pull-ups the second minute, three pull-ups the third minute... continuing as long as you are able.
Use as many sets each minute as needed.
Actual workout...Some were pull ups, some chin ups
1,1
2,2
3,3
4,3
5,4
6,2
7,2
8,1
9,2
10,3
11,3
12,2
13,1
14,3
15,2
Total time 14:30