I'd be surprised if you got more than 4 months net honestly. BBGs tend to be trendy, and tend to have poor retention rates among the average user. A lot of the ones you see for sale were people's project for learning php, and as a result they are poorly coded. Your revenue may be consistent for a few years, but consistent is not the same as growth... and since gangster games have been the hot trend over the last year or two, I personally view 'consistent' in a growing segment as being a negative.
If you were selling clients who subscribed to a service like webhosting or something else which had barriers to leaving and/or incentives to stay, I think 6-12 months net would be realistic and probably even conservative. Hope I'm wrong for your sake, but I've valued properties in the past and this was the general line of thinking.