Too much time would be when they start feeling neglected. If they are online playing WoW or something and you are here having fun, my guess would be it is no problem, but when you start to ignore each other or when one of you can not get the attention desired from the other, then it is time to have a talk and perhaps scale back the online interaction.

Something fun, if both of you are online, sending each other little sexy, flirty text messages can be a great way to get one another worked up