As the "Systems Visited" is the only live counter in the Exploration statistics, it's IMHO the only rough guide one can take.
Of course it depends on what is being scanned, but the ELITE Rank typically seems to set in after some ~7000-8000 Systems visited.
It'll be less when every single Object is scanned per System of course, and quite a bit less when i.e. Neutron/Black Hole farming is performed.
At least with Neutron/Black Hole farming, some made a pre-calculation based on existing known Credits/Scan values and set a goal on how many of them they'd need to farm. That seems to work in that specific case.
To track the actual progress, there's some 3rd party Tools that do that tracking (at least in terms of estimated Credits Value). With "Verbose_logging" enabled in the proper xml config file, such a Tool is likely the only way to get a somewhat accurate estimate.
Drawback : those tools won't work in retrorespect - meaning you'd have to run them at the start of your tour. They can't assess what you already hold in your Map Data Storage from before the tool was installed.