I highly doubt that anyone can give you a precise information on that.
It really depends on the course quality, the demand and the amount of effort you are willing to put into it.
I started in October 2010 and just played around with Udemy a little bit and created very different courses to try a few things.
One course started making 500-700 Euro each month (after month 3), some other courses havent even made 200 Euros in 7 months. Thats courses in german, i assume there's even more potential in english, but its obviously harder to beat the competition and gain attention. So it might take longer to make money.
Best regards,
Niels
