This cant be answered, especially not with the info you've provided, and extra especially not without a picture. At least with good info and a pic some people might throw in a guesstimate.
And 18/6? Is that the lighting cycle youre on? If so, you're not 30 days from harvest...
Edit: i see you posted a pic whike i was replying.... so ignore that part.
Dude your not even in the flowing stage....... 18/6 is veg lighting schedule. Its the same as having the light on 24/7. You need to turn the light off for 12 hours. then you have 60 days or how ever long the strain calls for (usually around 60 days)
Looks like you posted a pic while the above respondents were responding It's flowering alright. But just barely. Unless that's a 6 week (42 day) flower plant, which I doubt. Looks like you have at least 6 more weeks to go. Once the plant stops its flowering stretch, you should be better able to judge yield. You know, how many buds on the plant, how big and how dense they are, blah blah......
Looks like you're using CFL's ... you can expect around .25g a watt if you've dialed everything in well. You need to lst if you want to maximize yield there or you can expect far less than .25g a watt