For the first transfection experiment I carry out a time and dose curve. Usually I use the dose recommended by the manufacturer and the double and the half (or 2/3 of the recommended dose). And with all doses I perform a time curve using 12 h intervals.
I do that for both overexpression (miRNA mimics) and inhibition (anti-miRNA) and I measure miRNA levels by RT-qPCR.
Then I select the time and the dose with the máximum effect on miRNA levels.
You could also order miRNA mimic and inhibitor labeled for example with Cy3 (Ambion) and check the efficiency of transfection via flow cytometry, ciaoooo