This question is really dubious and it is not very clear what is meant by the time spent on forex.
If you meant, how much time we should spend on learning the market, then the answer will be as much as you can. Forex is constantly changing and there are lots of different tendencies which are changing all the time and a trader should keep up with the market in order to follow a trend. However, if you ask, how much time should we spend on trading, then it depends on trader. There are many opinions and approaches to trading and it is impossible to say which of them are objectively right or wrong. If a trader is a fan of short-ter trading, then he spends much time sitting in front of the computer watching the charts. However, there are also people who are fond of long-term trading and they open the trading terminal only several times a day.