Well it's going to be obvious if a coin is suspended,
a. no withdrawal
b. no deposit
c. no buying
d. no selling
There could be a lot of reasons why a coin is going to be suspended by let's say Binance. But I speculate that it could be of security reasons, maybe there was a hack that is going on or at least that coin has been compromise. So it might take days or months because Binance will resume everything on that coin.
Most of coins suspended because has security reason with their project and binance will stop immediately for withdrawing, deposit and trading until the coins project announce they are return back from security cases. I saw many time Binance usually suspended with withdrawing or deposit and coins have drastically difference price between one exchange with another exchange but back to normal price after releases suspending.
For releasing form suspended I don't think need longer time regarding has many kinds of coins suspended in Binance taking without one day back to normal for trading, deposit and withdrawal.