Flooding your subscribers with mailings is not a good idea, but neither is emailing them too little. Aside from missed opportunities, there’s a tendency for people to forget what lists they’re on, and if they haven’t seen anything from you for a long time, they may think it’s spam and unsubscribe you. Worse still, they may mark you as spam and put you on the naughty list with your mailing list manager.
I’ve found that three months is the longest you should go without emailing your list.
If it’s been that long, you should send out an email right now to reconnect with them. It would be great to send them some useful material (you must have created some in that time, right?), but at the very least send a pure greeting email, bringing them up to date on what you’ve been doing.
If it’s been much more than three months, make sure you’ve got something useful to send them, and talk about your plans for the future (ones you can deliver), such as having a regular mailing once a month (or whatever schedule) or an event you’ll be running for them shortly.