Although I see similar questions have been asked by different users before, but still I'm not able to find a satisfactory answer and I'm not sure if it's a bug specific to workspace analysis?
If I just want to see an overall view of avg.time spent on site, I see 'infinity' but if I break it down by months or some other dimension then I can the values being populated. However, it's also not the case, when we apply segment. Let me share with few screengrabs.
per definition, the "time spent" metrics are calculated against a dimension! since bothe time ranges (first table) and segments (second table) are not dimensions, it can't find any visits with that dimension (since there is no dimension) so any average is divided by 0 (equals infinity).
from a technical view I believe that the data request wants to check at a hit-level where a certain dimension exists. and since both "date ranges" and "segments" are no dimensions, the request will fail or return 0.