Nobody knows. There's no way to reliably test it.
I might be getting some hardware next week that I can run several instances of primecoin on, with 1M, 2M, 4M sievesize etc. Then I can do some statistical analysis on the results. But it will be one of those "the longer you leave it, the better the results" kind of things so it might take a while :
If you're using windows to run the tests I got some strange results. I ran several short tests and primecoin-qt always found blocks faster than primecoind and I mean MUCH faster. 300-600% faster. All of my primcoind tests only found 10 (I found that extremely strange) blocks and within that same time period I'd find 30-60 using primecoin-qt.
This was with various sieve settings. 500k, 1million 1.5 million. My test period was only an hour though.
1-3 days might be enough to get an average.