Trading requires time in a couple of ways. The first is the time dedicated to developing a trading system. This can be thought of as a one-off thing, but in reality it is more an on-going process. Once a system is in place, time is required in terms of monitoring the markets for signals, executing transactions, and managing positions. How much time all these different elements require depends on the trading system. The trading system, in turn, needs to take in to account the amount of time the trader has available.
How much time do you have?
With that in mind, the first question to be answered is how much time each day/week/month (whichever is most appropriate) can you dedicate to the various requirements of trading and managing a trading system? Different trading styles require different time focus. As a rule, the shorter-term the trading, the more specifically dedicated time required. A day trader, for example, runs positions which are opened and closed during the same session. This normally means a lot of time spent watching the market for entry and exit signals. An intermediate or longer-term trader who holds trades for weeks or more does not have to dedicate the same amount of time to watching the markets. He or she can usually get away with only spot checking from time to time. Of course there is a whole array of possibilities in between.
Is your time interupted?
At this point it is also important to consider distractions. There is a major difference between having 6 hours per day of uninterrupted time to watch the markets and having 6 hours of time during which you will be making and receiving phone calls, having meetings, and otherwise not being able to focus on the markets and make trades when required. In the former case one could day trade. In the latter, however, day trading would probably be a disaster as the trader would most likely miss important trading situations on a frequent basis. This sort of thing needs to be taken in to account.
Consistency is key
The basic decision one has to make is in what time frame the trader can reasonably expect to operate on a consistent basis. The individual must be able to do all the data gathering, research, market analysis, trade execution and monitoring, portfolio management, and any other functions required of her or his trading system. That means a trading time frame has to be selected which allows the trader to handle all of these duties without the kinds of disruptions which can cause poor system input from the user, and therefore poor system performance.