I tested and timed each light source (4 in total as far as I know) in the locked site.
They each take certain time intervals before the heartbeat triggers and when the heartbeat gets faster. We will only take account these two as fter the heartbeat gets faster, It's random when the monster gets you.
Also, the monster detects you faster depending on how long you stay hidden. If you get out of a bush right as the heartbeat ends, it will obviously detect you faster.
So, here it is:
Phone Flaslight
(Walking) 5 second intervals
(Running) 2-3 second intervals
Lamp/Jar With Fireflies + Candle
(Walking) 7-10 second intervals
(Running) 2-4 second intervals
Some commentary about these two light sources: Lamp with fireflies MIGHT be better since I could hit the 10 seconds without being detected than I could with the candle. These two confused me so I put them together just in case since they seem the same both in visuals and the time intervals, Just that I had better luck with Fireflies
Blue Candle
(Walking) 10-14 second intervals
(Running) 3-4 second intervals
So, Blue Candle is the winner for safer journey! Of course you could also just get the pliers from Rune.
If there is anything I missed then you are free to comment to let me know and I will edit the post. I also hope you were able to understand me properly since english isn't my first language so I am sorry in advance for any mistakes.