I think thats why this question would be best answered by someone who went through a well defined set of meditation practices to reach stream entry including following of the access concentration then insight type path. My path was a mish-mash of things/ 'kick all the doors in'/drill 100 wells type approach. It would be a long off topic ramble to write out here. Further, i never really knew what the hell i was meditating for anyways.
Yes, big change in concentration after stream entry both on and off cushion.
On cushion: hadn't meditated with any frequency for maybe a year before stream entry because it was uncomfortable (with hindsight i was just hitting dark knight and thought something had gone wrong). Post SE concentration is much more laser like (and lots of fun now to boot) and i go through the insight stages and jhanas through to fruitions, although i am still sketchy on which jhana or dark night insight stage is which most of the time. I'm not a technical meditator due to my lack of formal background, but am working on things because its fun, a supportive internet community and there is still work to do (ie. still becoming).
Off-cushion: 2 months after SE i returned to engineering studies after number of years break. Did not know what SE was, only that something significant had occurred that shook identity in a deep way and had a number of odd side effects. Went from middle/fair/average student to top of year student, attributing this to both maturing with age and significantly less of 'my' stories/proliferations/schemes getting in the way. i understand now that the reduced mental proliferation is due to SE.
I am still unsure what access concentration is referring to as people seem to describe it in different ways. If you use the definition found in Daniel Ingram's excellent book Mastering the Core Teachings of the Buddha (http://www.interactivebuddha.com/Mastering%20Adobe%20Version.pdf) i am unsure about when i first managed to do this. i would take a punt and say a couple of months of trying to meditate 'every now and then' without a formal plan. i would say SE occurred around 4.5 years after becoming interested in this type of stuff. i stress that because i am a westerner from a country town with conservative anglican parents, asking questions like 'who am i apart from all these labels and names?' and 'what is this energy/well of bliss that can be willed from the base of my spine?' would have gotten me some strange looks, let alone following an eastern tradition.
Recalling a little more now i remembered when i first started meditating after reading about counting breaths in a meditation book. I think i lasted about 10 minutes before becoming fed up with it as this seemed like a long time to be sitting still doing nothing. This improved with time (so there must have been improvement), there was one point maybe a couple of years before SE when i managed to do 40 mins a day for a few months...probably hit DN constantly after that and stopped.
