In your situation, when you export the song in FL Studio, you can always look at the Time Panel in the top of FL Studio which shows you how far along you are in the song, then export the song to around 60 Seconds.
Pro-Tip: Clicking the Time Panel allows you to switch between two modes:
If you right-click the Time Panel, you will get a drop down for more info on the options.
How to find beats per minute by manually counting:
You can try and count how many beats in 60 seconds for the song, but that takes a long time. A popular shortcut to find bpm in a song is count beats for 30 seconds, then multiply by 2 to save time..
To save even more time, you can just count how many beats in 15 seconds, then multiply by 4.
When you count your beats, you have to minus 1, because the very first count is 0.
For example, if you count 30 beats in 15 seconds, it will be 29 beats, so multiply that by 4 for 116 BPM. But most often you may be off a beat (or in between a beat) when counting.. So in this example, even though it shows 116 BPM, most songs ended up being 115 BPM.
Another alternative is to use an online beats per minute counter where you can click or tap to the song’s timing.
FL Studio also has a built-in beats per minute counter. This is found by right-clicking the Tempo in FL Studio, then selecting Tap from the drop-down menu.
Simply click the tap until you get your desired beats per minute (BPM).
Does that help at all?
In your situation, if you’re exporting the song, you have full control on how long the song will be by looking at the Time Panel to visually see how long the song is!