Hi Hong, this really depends on the gene you are targeting with your lentivirus. In general I tend to add my virus at T=0, feed (add more media) at T=24hr and then test knockdown at T=72/96hrs with qPCR or western blotting. For the majority of my lentiviral transductions, any knockdown is observed within this timeframe.
For LV transduction, the longer you leave the virus in contact with the cells, the better your transduction will be. Christopher gave a good guide above, normally I would see some expression at 24h and maximum expression at 48-72 hours.
However, those timelines are for gene expression, not transduction itself. If for various reasons your transduction must take place over a very short time, 6 hours will be enough time to get fairly decent transduction dependng on your cell type.
There is a Lentivirus Webinar coming up that may able to help address your concern and any additional questions.
Title: Knockdown, Knockout, Validate: Lentivirus delivers payload in vitro and in vivo Date: Wednesday, May 16, 2018 Duration: 60 minutes Times: 10:00 AM (CST) Speaker: @Christy Hoffmann
What you will learn: Lentivirus biosafety features and best handling practices Flexible customization options to fit your needs Techniques to incorporate lentivirus into your gene studies Insight from a leading Lenti manufacturing expert