According to Wikipedia, "adults of good average fitness average between 50 and 150 watts for an hour of vigorous exercise. A healthy well-fed laborer over the course of an 8-hour work shift can sustain an average output of about 75 watts."
On that basis, let's assume you can average an output of 70W while cycling.
Your 25.9V 5.5Ah battery stores ~ 142Wh of energy, so if we assume 50% efficiency in converting all your cycling energy to electrical energy (i.e. you are using a static cycle, not one on the road) it would take ~ 2 x 140/70 = 4 hours of continuous cycling at maximum speed to charge the battery fully with that alternator!