Well, an iPhone 5S battery is 6Wh, an iPhone 6S is 6,5Wh and 10Wh for the Plus model, and your higher end e-bike battery is typically 400-500Wh depending on the model.
The 6S 6,5Wh battery claims to last about 8 hours of average use, so lets halve that for your use, which means 4 hours on 6,5Wh = 1,625 Watts of draw. So your phone will drain your 400Wh e-bike battery at a rate of 1,625 Wh per hour.