The formal start of the election campaign is now just over a month away.

Four weeks on Monday will see the dissolution of parliament: the official start of the campaign, and the end of the 2010-15 parliament.

I know it feels like campaigning has been going on for weeks, if not months. But not until dissolution takes place on 30 March will “purdah” begin: the period of time when special rules restrict the activity of civil servants and the business of government in the run-up to polling day.

All facilities provided by the House of Commons to MPs in Westminster are no longer available to them from 5pm on the day of dissolution. MPs have access to parliament for only a few further days in order to clear their offices, but by that point they are no longer technically MPs. Ministers retain their ministerial titles after dissolution, but those who were MPs can no longer use the MP suffix.

The official campaign will begin at an earlier point in the electoral calendar than in recent decades.

This is thanks to a change in the law concerning the minimum number of working days required for the campaign, which has increased from 17 to 25.

Add in the weekends, Easter and other bank holidays, and we’re set for the longest official campaign in modern history. From the date of dissolution to polling day is a total of 37 days. Here’s how that compares with the elections of the past 40 years:

Legislation has always governed the length of this period, so it’s not surprising to see so much consistency. There’s more variation to be found in the length of what I’m calling the “unofficial” campaign: the period of time between a prime minister announcing the election, and polling day itself:

Remember how long things felt in 1997? It was speculated at the time that John Major was hoping such a mammoth campaign would make it more likely Labour would slip up. But while the opinion polls did narrow, the outcome was never in doubt.

1979 was slightly atypical, as the campaign unofficially began without a formal declaration from prime minister Jim Callaghan. Instead it was triggered by the Labour government losing a vote of confidence in the House of Commons.

I’m guessing David Cameron will mark the dissolution of parliament with some sort of formal televised address or a speech in Downing Street. He’ll almost certainly want to do something, given there’s no longer any point or need for him to go to Buckingham Palace.

Before that we’ve still got four weeks of business left, which includes the Budget on 18 March, four more sessions of prime ministers’ questions, plus the usual rush to push through any outstanding legislation.

And then, come 30 March, 650 MPs will be made redundant, parliament packs up, and it’s over to us.