The rule comes from Article 85 of the Constitution, which provides that the President shall from time to time summon each House of Parliament to meet at such time and place as he thinks fit, but so that six months shall not intervene between its last sitting in one session and the date appointed for its first sitting in the next session.

So the maximum permissible interval is six months.

Two consequences follow, and they are what the provision is really for. The first is that Parliament must meet at least twice in every year, since no gap may exceed six months. In practice it meets three times - the Budget session from February to May, the Monsoon session from July to August, and the Winter session from November to December.

The second is that the executive cannot govern indefinitely without facing the legislature. Since the President acts on the advice of the Council of Ministers, the government effectively decides when Parliament is summoned; the six month limit is the constitutional check on that discretion, ensuring the House cannot be kept away from scrutinising the government for long.

Two points of precision are worth noting. The gap is measured from the last sitting of one session to the first sitting of the next, not from the date of prorogation. And the rule prescribes a maximum interval rather than a minimum number of sitting days, so it guarantees that Parliament meets, not that it meets for long.

The same six month rule applies to State Legislatures under Article 174.

Hence the interval between two sessions of Parliament must not exceed 6 months.