With Vision (currently) you are not billed for minutes at all (see another thread about how I wish this were an option...).
Instead you are either billed for bandwidth (kbytes transferred), or, if you have an "unlimited" Vision plan, which is what most Vision users have, you're just billed a flat rate (free or $10 or $15/month).

So, for most users, you can poll constantly, as you suggest, and it will not increase your bill. There is no need to explicited disconnect if you don't want to. However that is not the same as being "always on". Generally, if the Vision connection is idle for a certain amount of time, it times-out and the connection is dropped and you must re-connect.