It depends on what power you can produce and the efficiency of the equipment you use to convert that mechanical power into electrical power.
As a moderately fit 50+ cyclist, I can produce over 200 W for five or so hours, 300 W for much shorter periods, and levels around 1 kW for less than a minute.
A commonly quoted efficiency for electrical generation is 80%, but lets assume 75% efficiency to be just a little conservative.
200 W X .75 = 150 W
300 W X .75 = 225 W
If it's just for 15 minutes, we can take the higher figure. 15 minutes is .25 hours, so we have
225 W X .25 h = 56.25 Wh
So that would run a 100 W load for half an hour, provided the remaining parts of the system were 90% efficient.
Mobile phone battery capacities vary. Some are advertised at over 2000 mAh, and given a voltage of 3.7 V, that's
2 Ah X 3.7 V = 7.4 Wh
On these figures that 15 minutes of pedaling would charge a mobile phone seven times.
One limitation will be the efficiency of conversion of the electrical power from one voltage to another.