Hi Mick,
In reply to your message, I would say (as people have said on here) that getting enough business is they most difficult bit, as with any new venture.
If you have a decent client base for windows then that would be a good starting point, although that would probably only produce some of the work you need. If you are using it as an add on to your windows and you don't spend too much to start with, then its pretty straight forward, just let you customers know that you are now providing this service, as well as the other work you do for them.
You would need to advertise/leaflet drop etc. do get some decent levels of business I feel.
As for how often, I would say every 12 months, some only have it done every 24 months, not done any 6 monthly.
All in all, as others have mentioned, it is worth it as an add on and see how it goes before you spend a fortune on it. It will know doubt depend on the area where you cover and what competition there is in that area etc...
Hope that helps
Cheers
Steve