The rule of thumb
For ordinary cargo, an e-way bill is valid for one day for up to 200 km, and one more day for every further 200 km or part of it. Over-dimensional cargo has a shorter distance per day.
When a day ends
A day is counted to midnight of the day after the e-way bill is generated, not 24 hours from the minute you made it. An e-way bill generated at 11 pm and one generated at 9 am the same day run out at the same time.
The midnight trap
Software that counts days in UTC instead of Indian time gets the date wrong for anything generated between midnight and 5:30 am, and shows a validity one day short. OSuite works out the date in Indian time.
When goods are late
If the goods cannot reach in time, the validity can be extended on the portal within eight hours before or after it runs out, giving the reason and the remaining distance.
General information, not tax advice; rules change, so check with your accountant for your case.