When is the best time to request pac code and switch?

[chipperhead]

I'm currently on Sim only and swap every 12 months to get the best deal. However I'm never quite sure when's the best time to request my pac code and then sign up to a new plan.
I'm with vodafone and my agreement ends on 04 Mar 2020.
Should I be requesting my pac code today as it's one month before?

 

[chipperhead]

Generally quite quick to transfer these days and I think the pac code lasts 30 days, wait till nearer the time?

Don't I need to give you 30 days notice though?