Update. . . . . .
To date (3-18-2011), there have been 39 replies to this thread. Note that not everyone was able to give their response in hours or days. Nevertheless the following numbers were obtained:
1) 22 people found their first their silver coin within an average time frame of 50 hours of hunting.
2) 9 people (including daddydigger, amxfred, dustyloins, fred sanford, diggermagoo, mark v, bowman 64, Me&myBoy, Silver Eagle) found their first silver on their very first hunt!
3) All but 3 people found their first silver coin within 6 months of hunting.
4) All but 2 folks found their first silver coin within the first year of hunting.
5) Neil Diggit gave up MDing for a while for girls (sorry Neil, couldn't resist that!)
So this very unscientific study appears to indicate that you could be lucky and find a silver coin almost immediately, but it is more likely that you will have to work the ground for up to 6 months or so for that first silver.
Happy Hunting!