You can continue with the incubation for the 24h

Some research uses different times to determine the final time most cells will die Maizan Mohamed Arif Hussain Dhifaf Zeki

The optimal incubation time in anticancer activity study depends on the drug ‘s mechanism of action and the cancer cell line you use. For routine assay in most cells, a 24 hour incubation with one doubling time of the cells in many cases is sufficient to see both survival and anti proliferation effect.

If your inention is more on the anti proliferation effect instead of cytotoxicity, a longer incubation say 48 and 72 hours are needed. A good experiment is that you set your assay at least at 24 and 48 h incubation.

You can read your IC50 after 48 and 72 hours

Normally, I will first set a time gradient of 12/24/36/48/60/70 hours, and then reduce the time interval based on the experimental results.

You can read drugs effect on cancer cell after 24,48 and 72 hours

Some compounds, you can only observed IC50 after 72 hours incubation.