I kept this to TOS/TNG canon.
NX-01 - Retired in 2161, it's unknown when the presumptive replacement of Daedalus class vessels launched. Presumably, their retirement marked the start of the next class, so it's unlikely there was an overlap. We know of no ships named Enterprise in this class.
1701/1701-A - Assuming that the Excelsior class was the replacement for the Constitution class (and no other classes count), the overlap starts in 2285 (where we see the class prototype) and ends in 2293 when the 1701-A was retired and the 1701-B was launched. So 8 years in this case.
1701-B - We don't know much about this Excelsior-class ship and when she was lost or decommissioned. We only know it had to have not been in service by 2344, when the 1701-C was lost. Since there's only one Ambassador-class in canon, we know nothing about when these were launched (non-canon says 2325).
1701-C - Since she was lost in 2344, there was no known overlap with the Galaxy class (2360+)
1701-D - Launched in 2363, the Enterprise-D served until 2371, when she was lost to a rogue Klingon Bird of Prey. We don't know a lot about the Sovereign-class, but they were in active development in the early 2370s (1701-E was in service by 2373), so it's possible that there was some overlap of 1-2 years. Given the rapid commissioning of 1701-E, this seems likely.