How often should I drain my bokashi?

You should drain off the leachate from your bokashi bucket at least every other day. 

Once the "tea" has been exposed to air, it should be used within hours, or it may start to putrify. It is safe (and even beneficial for your sewer) to pour unneeded bokashi juice down the drain.  

If you don't want to bother with draining off "tea," you can use a bucket without a collection reservoir and, instead, add sufficient dry material (shredded paper, wood shavings, etc.) at the bottom of the bucket to absorb the juice being produced during the fermentation process. It will likely require some trial and error before you learn how much dry material you need to cope with the leachate from the unique composition of your kitchen waste.