Dirk wrote:the period for naturalisation is 5 years if married to a British citizen
The period of naturalisation is 3 years if married to a British citizen.
However, it now takes a minimum of 5 years to reach settlement (ILR or PR), which is a prerequisite for citizenship.
As the spouse of a British citizen, your spouse would be able to apply for naturalisation directly on attaining PR status. She will need to submit the same proofs if she were making both a PR claim and an application for citizenship simultaneously.
Dirk wrote:I don't want to jinx this by thinking about 5 year from now
Assuming that the UK remains in the EU...
Assuming that the UK does not manage to water down either the SS route or the freedom of movement of family members in general...
Assuming that if the above two are true, that SS route holders are grand-fathered into the citizenship route (remember that citizenship rules are completely unrelated to immigration rules)...
Far too many assumptions to plan ahead with, but I commend you for having the foresight.