I think it's fairly common knowledge that a lot of older Test scorecards don't show balls faced - only minutes at the crease, which makes it difficult to properly assess a batsman's actual strike rate apart from 'he was fast' and 'he was slow'.
I do have a proposal which I think our statistical wonks could spend many hours developing though.
If we assume the average over rate is 15 overs per hour, then it would make sense that there are 90 balls bowled an hour. Assuming nobody is deliberately farming the strike, each batsman would face on average 45 balls per hour.
Ergo, if you spend an hour at the crease you face about 45 balls, or 3/4 balls per minute, everything else being equal.
Let's test this theory a bit.
Hanif Mohammad's match saving 337 took 970 minutes, which this farcically simple model infers means he faced around 727 balls. Given Hanif's reputation as a slow scorer and The context of the innings (saving a match while following on) I'm not sure he scored those runs at a relatively normal SR of 46. Moreover the total run rate for the innings - for which Hanif was responsible for over half of - was only 2.06, which means a team SR of 34. This model doesn't really add up.
Alright, let's try again.
This famous Ashes Test is remembered more for Benaud's ingenuity than this innings by Ted Dexter, in which he scored 76 in 84 minutes - implying that he faced 63 balls. This doesn't seem unreasonable given that the innings itself was described as 'dashing', 'cavalier' etc.
Funnily enough, in an Ashes series 23 years prior in 1938 they were thoughtful enough to actually count balls faced.
Stan McCabe's legendary 232 at Trent Bridge was off 277 balls. He spent 233 minutes at the crease. While he was obviously farming the strike for the last few wickets, the model says it took him 176 balls. By comparison, Len Hutton in the same series scored his 364 off 847 balls. He was there for 797 minutes, which means the model is wrong again.
So how could one develop this model in light of so many unknown variables like time spent on strike and over rates?