Anyway, to answer the question, I reckon 2-5 years minimum.
Most of the reasons have been covered (quality batting lineup, deep batting lineup, best bowling attack, best spinner) and with the exception of South Africa, I don't see a side that can topple us in the near future.
South Africa have their issues; Alviro Petersen as an opener, their number 6 batsman, 3rd seamer and spinner all being short of Test class means that they're reliant on a handful of, admittedly very good players. We saw what happened when they played Pakistan in the UAE and Steyn was rendered ineffective - a 0-0 drawn series. England, for some reason, seem to be the one side that a lot of the South African players really struggle against as well. South Africa also have their political issues - the obvious partner for Smith at the top of the order is Jacques Rudolph, yet the quota policy means that to select Rudolph they need to find another black, coloured or Asian cricketer elsewhere. I don't think a side can truly have a run at number 1 when they're prevented from picking their potential strongest XI.
India - with the exception of Praveen Kumar and to a small degree, Ishant Sharma, this tour was a forgettable tour for all their "bowlers". Zaheer Khan, their one truly Test class bowler, is now injured for 4 months, and at 33 years of age and with his injury record, whether he'll ever be a force in Test cricket again is a legitimate question. Batting wise, you'd have to assume that Dravid, Tendulkar and Laxman will all call it a day at some point over the next few years, and the fact that India have failed to find a settled, successful number 6 despite Ganguly's retirement coming 3 years ago has to be a worry. If they can't manage that correctly, how are they going to cope when their greats retire? India's recent rise to the top has come in spite of, not because of their board, and Ishant's declaration that he'll undergo ankle surgery after the Border-Gavaskar series is a serious indication that lessons haven't been learned from the recently concluded series. They'll remain a tough proposition in India, but I wouldn't be suprised if an overseas Test win is a long time in coming for India.
Australia - have an upcoming transition phase as Ponting and Hussey will need phased out in the next couple of years. They've still got question marks over their batting; they've not yet addressed their weakness in that suit, although the customary once a series ridiculous batting collapse has yet to occur in this series. By the time the next Ashes rolls around I'd expect them to have recovered to number 3, but I feel that the next few years for Australia might mirror England's 2005-09, as an inexperienced batting lineup finds its feet in international cricket. It's easy to forget how much England's batting lineup transitioned between 2004 and 2009; of the side which started the 2004 series against New Zealand only Strauss (on debut) and Trescothick actually started the 2005 Ashes series; Nasser Hussain, Mark Butcher and Graeme Thorpe were all pensioned off over the space of a year. England's batting lineup which emerged has gone on to prove their quality, but it came at the expense of years of inconsistency from the likes of Strauss, Cook and Bell. I reckon Australia might experience something similar for the next few years.
Sri Lanka - will remain a tough proposition at home but are now, barring a miracle tomorrow, set to extend their winless run to 10 Tests since Murali retired. Their bowling attack won't see them win too many Tests overseas, and unless a couple of gun spinners are unleashed, they'll struggle to win many Tests at home.
The one side who might suprise the rankings are the West Indies - they've actually got the makings of a very good bowling attack (Roach, Edwards and Sammy all average under 30 in the last 2 years) and they've got some talented, if raw batsmen. Their batting recently has been a complete let down, but at the moment they've got a few players playing County cricket - if the likes of Darren Bravo and Kemar Roach have a successful couple of seasons in county cricket then it should help them massively develop their games and could lead to a resurgence from the West Indies. I wouldn't be at all suprised to see them push towards the top 4 in a couple of years.
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