there are different options how to measure this
1) track time on client side:
just write the timestamp at "cart add" to cookie/localStorage and calculate the time diff later at "checkout start"
2) use segments and "time spent"
never tried, but you could setup a few segmentd and calculate the difference on existing data
a) segment "hits until cart add" and metric time spent
b) segment "hits until checkout start" and metric time spent
c) overall segment "visits: cart add exists and then checkout start exists
having the overall segment (3) you can take the diff of (1) and (2) to get the time.
remark: for "hits until" use the rockstar tip from audrey salerno, link on this page: https://adobe.ly/aarockstartips