... I believe around 1ltr per minute is an often quoted rate for small scale systems, so bearing in mind you may "vorlauf" the first few litres (lauter out some wort and recirculate it back into the top of the tun) until the wort runs clear, then you're probably looking at around 30-35 mins
... but ultimately, the process becomes a pay-off between efficiency and time and at the scale of us home-brewers (and even for some pro-brewers) the cost of a bit more grain can seem minimal compared to the time taken and patience needed ... and/or, as Cheshire Cat's experiences may well be an example of, it can be easy when trying to speed up a fly-sparge to compact the grain bed, not enough to stick but just enough to create channels through the grain, so the sparge water comes into contact with hardly any of the grain, leading to poorer efficiency than could have been achieved by batch sparging
... Yes ... but then there are all sorts of reasons why your OG might have been low, including those related to less efficient fly-sparging as discussed above ... do you usually hit your OG targets?
Cheers, PhilB