Hello Fatemeh Akhlaghi

First, I want to know if it is necessary to perform the test in 24h AND 48h AND 72h? Or it is possible to perform it just for one time point or two?

Yes, it would be better to perform the test for three time points namely, 24h, 48h and 72 h. You may not be knowing the mechanism of action of your test compound. The test compound may not show any effect after 24h and therefore longer time points may be required (say at 72h). It also depends on the cell type used in the MTT assay. So, if you have no previous data to refer to, it would be better to try each time point, and then report the IC50 values specific for each time point.

Second, is there another reference which contains the time points of 1, 3, 5 days or 2,4,7 days (these are the time points I saw in papers)?

Yes, please refer to the article attached below for three time points namely, 2,4 and 7 days.

Article In Vitro Biocompatibility and Endothelialization of Novel Ma...

The viability is affected in a dose- and time-dependent manner. For instance, prolonged exposure can cause inhibition of cell proliferation even at low concentrations. Sometimes, the cell viability may not be affected even on the 7th day which could possibly be that no serious damage is done to the cells like damage to the cell membrane but only may cause decreased enzymatic activities or altered gene regulation.

Best.