When Sanhedrin existed, each month had to be sanctified by Sanhedrin after testimony of 2 witnesses (or automatically by Sanhedrin if no one appeared by the end of the 30th day.) Refer to Maimonedes Hilchot Kiddush Hachodesh for details.
Since each month was individually sanctified, I assume that it was possible to have several months each with 30 days or several with 29 days, unlike the alternating lengths that we have for most of the months in our current fixed calendar.
Was there any limit to the number of consecutive months having the same length? For example, in a leap year having 13 months, would it be possible to have a year that was 390 days long if all the months had 30 days? If not, what was the maximum number of months that had the same length?
A long run of one type or the other would eventually become obvious to anybody who witnessed the new moon, but if nobody witnesses and comes to Jerusalem to testify, the Sanhedrin could conceivably have a long run of default-30-day months. Is there a halachic limit?