I could see UKip winning at least a seat at the next General Election, given the increasing euro-scepticism within the British electorate, and it is still far from clear that Labour will win a majority at the next election, so both of his criteria for retiring/resigning are not automatically going to be fulfilled.
Personally, I think UKip are still too reliant upon Mr.Farage as the acceeptable public face of their organisation for this to be considered a serious "pledge".
He certainly has a charisma that the electorate responds to, and whatever else you think about him, he has helped to shape and focus the debate on Europe - what it is for, what the benefits of membership are, what the costs of membership are - all of that, which is a good thing. People should be better informed about what the EU is and have an opinion on what they want it to be.
I also suspect they will do pretty well in the upcoming Euro-elections, and they will return more UKip MEPs. It bothers me though that UKip MEPs refuse to engage with the euro-parliament, rarely bothering to even turn up and attend the debates. This disengagement with the political process might play to their ideology and their supporters, but is not pragmatic or in Britains best interests whilst we are still members, and means that such MEPs really are getting money for nothing -money that there is a suspicion they are using against the rules, by funding UKip back in the UK.