Three possible culprits here.
1. Battery
2. Alternator
3. Regulator
Once you have a good charge on the battery, how long will it keep the charge if you do NOT start the bike? (Try to do a trickle charge on it, not a quick charge.) If it will hold the charge for a couple days and start the bike normally, then the battery is probably not the problem, but to rule it out, you can charge it up and take it to an auto parts house or mechanic/shop (auto or m/c) that can load check it to be sure.
If the alternator isn't putting out, or the regulator isn't regulating current to the battery, the bike will run off the battery until it has drained it to the point it can no longer provide enough juice to run the ignition. Lights & horn will work, but will gradually get weaker as the battery drains.
If they (alt/reg) are working, but the battery is weak, they will easily provide enough juice to ride the bike and run all the electrics indefinately, but since they only put out juice when the engine is running, there'll be no juice for the starter.