Hi Sadegh,

if you are using high titers lentivirus, use an MOI of 10 or 20 and add a compound like polybrene to your medium you will have a near 100% of transduction.  

You might be able to select your cells with puromycin (or else) depending on your set-up to get a ballpark efficiency estimate. Otherwise, you could conduct a control experiment encoding for a GFP and resolve your efficiency using a fluorescence microscope straight from your culture plate. 

Best,

Nicolas

Nicolas gave a very good answer. I will just add that it will strongly depend on your cell type. In standard inmortalized cells you can easily have near 100% transduced cells with an MOI of 10-20. However fo some primary cell types like HSCs monocytes, T cells or iPSCs, you will find very hard to get over 60% (although you can also get near 100% uisng higher MOIs)

Assuming decent viral titer, you can easily achieve over 90% transfection efficiency in most immortalized cell lines. With hematopoietic lineage cells it may be lower. In our hands we have gotten close to 100% infection efficiency in mesenchymal lineage cell lines, whereas macrophage lineage cell line we have only gotten close to 50% efficiency. With primary cells, it again depends on the cell type, but with a high titer you can get close to 50-70% efficiency.

Having said that, expression of the desired protein will depend strongly upon the promoter used, and the protein itself. It is very important to choose your promoter correctly.