Thank you for your help. This solved my problem:
base search|convert timeformat="%Y-%m-%d %H:%M:%S" mktime(time*)|eval total_time=((time_complete-time_initial))|rename total_time as total_time_avg|eval "Average Time to Complete"=case(total_time_avg<=120,"0-2",total_time_avg<=300,"3-5",total_time_avg<=600,"6-10",total_time_avg<=1200,"11-20",1=1,">20")|fields - total_time_avg| timechart count by "Average Time to Complete"
I would convert this to an answer but I don't have the permissions.
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