I've been in herceptin on/off for 2 1/2 years and my echos (I've only ever had one gated heart pool scan) are usually 3 monthly. When on trials they were strictly 3 monthly but I haven't had one for nearly 6 months so I have a slip for one now.
I'm surprised to hear that your onc is waiting 5 months - but wouldn't be concerned. Have you asked why? The only reason for my current gap is we have been distracted with other things and forgot about it.
We may be hard to contact but if you are concerned you are within your rights to contact. In the public system they often won't give you herceptin if you haven't had s heart scan every 3 months.
Good wishes,
A x