That is a network-specific action, again it has nothing to do with mIRC. Obviously on the network you use channel registration needs to be checked by an administrator - because that involves human intervention it could be any length of time. If they are a serious network then I doubt it will take longer than 48 hours or less. That is not a definite amount of time though.
Basically, all you can do is wait, but it shouldn't take an excessively long amount of time. Again, you may want to ask for advice in the network's #Help channel or check out the network's website.
Happy chattin'
Regards,
