I guess it all depends on a couple of things. How deep is the water that you're fishing and what color is it.
I fish senkos in clear water lakes - a good 10 feet of visibility and in mostly shallow 1 to 4 feet of water. Once my Senko has entered the water I watch the line like a hawk 'cause 90% on the strikes will be on the fall. If I don't get bit on the fall I wait about 5 seconds then begin a real slow retrieve, raising my rod only about a couple of inches to get the bait off the bottom and falling again. I repeat this until my line is halfway back to the boat then I start over. So I'd say it takes me about a minute to work a senko per cast. Other people will say to let it sit there onthe bottom for a longer time - and they're right about that too, but this is my preference - and it works well for me. A word of advice here: as good as a senko is at catching fish just about anywhere in the water, your odds will increase big time if you toss them near, or into cover, such as shade, holes in weeds, stumps, logs, rip rap, and so forth.
Hope this helps. Dan