Rowan, firstly and foremost in my mind, so sorry to read of Libby's passing. There is solid evidence to support the view that the grief of losing a pet is no less comparable to that of losing a relative or close friend.
There is no easy answer to your question, yet the very asking of it suggests to me that you might be feeling guilty about the idea of having another feline friend in your life and you wonder when "too soon" is "too soon". There is only "too soon" when you create it. Here it's more about processing your loss and the feelings associated with it.
If you're in the space of saying, "I really want another cat in my life.......but I feel guilty...." then that's two conflicting emotions. You're ready when you're ready, be it today, next week or next year - the differentiating factor is whether you allow any negative feelings to override positive ones, e.g. allowing guilt or shame or feelings of selfishness take over the desire to share your life with another living creature.
Why deny yourself that joy?
I posted on here back in 2011 when my Matilda died suddenly of internal organ failure, a few days after my (now ex) wife had accidentally run her over with our car. The grief of that loss hurt badly. It was, I think, over a year before we took in another cat (we had three at the time, so when we took in a new one, it wasn't to 'replace' Matilda but to add to the feline family we already had).
If you want to go to PMs, my partner is a trained peer-counsellor who actually specialises in pet loss grief, believe it or not. She was head of Estonia's equivalent to the RSPCA for many years.
Take care mate.