Dating St. Stephen's death precisely is very difficult but we know it was 45 A.D. or earlier.
It is widely held that that the Jerusalem Council (Acts 15) was held between 50 and 55 A.D.
Paul writes that he went back to Tarsus or Antioch for about 3 years, after he first met with the Apostles shortly after his conversion recorded in Acts Chapter 9.
Steven's stoning seems to have occurred shortly before Saul (who became Paul) was sent to Damascus and was converted on route but there was likely a short period before this happened so, working backwards, we can assume it was between 33 A.D. and 45 A.D.
Let me clarify this in more of a timeline manner to make it easier to follow.
Acts 15: Jerusalem 50 A.D. to 55 A.D.
At least 3 years prior, Paul met with Peter, James and John. As we know Paul wrote in Galatians that after that meeting, he went off to study and pray for 3 years in Arabia. The text in Chapter 9 also says Paul stayed some time in Jerusalem preaching. So this is sometime after Acts 9.
Chapter 8: Christians flee Jerusalem to places like Sammaria immediately after Steven's stoning at the end of Acts 7 and the Gospel spreads to Jews outside of Jerusalem . . . that's likely when it reaches Jerusalem.
So Steven's stoning could have been as late as the late 40s. Again, we can't know for sure but it certainly happened several years before the Jerusalem Council (Acts 15).
The first few Chapters of Acts tends to lack more details than the latter, as Luke was writing from second-hand accounts. He didn't join Paul for many years and he starts writing in the first person, so the first few chapters are compressed.
That's the best that the biblical text seems to offer. At least that's what I can recall off hand, without actually doing a more in depth study.
I'm not sure if there are any reliable extra biblical sources that can narrow the date down.