Here are a few things I tried as I was in the same situation about 10 years ago.
I first of all figured out what time frame I wanted to trade because that told me how much time I was going to need to trade each day.
You have that figured out already, (I also use the 4 hour and Daily charts for my swing trading method)
Once I determined what time frame. I started looking for the technical trade setups and how often they occurred.
I literally setup a video camera in front of my screen and recorded my results.
I moved one candle at a time on the time frame I chose and looked for the technical set up pattern,
once I found the candle that provided the signal, I recorded on paper and video the entry price. Then I continued to move each candle until it became profitable.
At first, I wasn't sure how long to hold the trade but after a few weeks of doing this, I started to see a pattern that repeated based on the technical setup, (example, a break out technique usually resulted in more profits than a re- entry in a trending market)
By testing and using this process, I got pretty good at determining how long to hold a trade open. this definitely included price on the chart not the length of time.
I also considered the average ranges that the pair travels in.
This may seem a bit confusing but if you just pick a starting point to begin the process, it will work itself out through your analysis and testing.
if you want more tips how I put this testing method together and details, let me know I am happy to share