Every 4th week should be good. Just make sure to make your recovery week just that, a recovery week. Also, make sure you rest days during your build are rest days. Either do a small amount of active recovery or if that feels like it will be a struggle, then just take the day off.

Recovery weeks are of great importance. At the start of the build phase consider four weeks of build, a recovery week then hit it for three weeks followed by a recovery week. Your IM training plan mindset has to remain flexible. If your resting HR is up, not sleeping well, are tired and not looking forward to the next training session you have to be comfortable adjusting the plan to fit your body. Age is a factor that has to be considered too but you can be old and tired or 31 and tired. The idea is not to fret and not get injured. You can't be prepared for IM if you don't train and you can't train if you are injured. Finding the balance is sometimes the real trick.
Good luck and don't forget to have fun with the training.

I do three or 4 depending upon my schedule.
I mix in a lot of events into my traning.
If there is a longer event coming up I will try and work it so my recovery week could be my taper week for the event, or my recovery week after an event