OK, so lets run some quick numbers...

Assume a meteorite of that size arrives on Mars every week. So far, Oppy has travelled about 1 km, and has explored a swath about 200m wide. In that time, it has found two suspewct meteorites - one a secondary (a fragment of Mars that didn't reach escape velocity) and one a primary.

Mars has a total surface area of 1.29E08 km^2, and Oppy has studied 0.2 km^2.

The ratio of these is the time required (in weeks) to accumulate an average of one Meteorite in the studied area. Divide by 2 and convert to years: 6.4 Million Years.

That's a tad younger than I expected, but to refine the number we need to look at size distributions - you quoted the figures for a meteor pre-impact of 3m or less. We need to convert that to the delivered size at the surface, and match it to a 30-50 cm meteorite.

Because the size distribution of meteorites is heavily weighted towards smaller ones, most of the arrivals in the sub-3m class are actually also sub-mm! So we probably neeed a couple of orders of magnitude weighting to get rocks of this size. That's an educated guess and I'd happily entertain a more rigorous calculation based on size statistics.

Anyway, the exposure age of the surface (or the lifetime of the meteorites before destruction) is then around 600 Ma. The factb that survival of the meteorites is an issue is suggested by the fact that we've found no primary stony meteorites, one secondary, and one iron. Irons should only be a few % of the total, unless erosion is skewing the statistics - which I strongly suspect is the case.

You could come up with rather different numbers if you tried, but remember to try to justify your assumptions with what we know about Mars...