If there were pesticides I would think they shrimp would die pretty fast. Maybe a week?
Why do you think the plants might have pesticides?
If you think they do, I would put them in a quarantine tank with some activated carbon in the filter and swap it out about twice a week for a couple of weeks, and do some very large water changes. This will dilute and remove the pesticide from the water, and the action of the water movement against the plants ought to remove the pesticide from the surface of the leaves.
If you think the plants were treated with a systemic pesticide I would not use them in the tank, but might take cuttings, grow these out, then take more cuttings of the new material.