In the sacrament of penance the penitent is only **required **to confess all mortal sins in kind and number, and to have contrition and purpose of amendment. (
canon law 987,988).
So, you are not formally required to state the length of time since last confession. If you omitted it completely, and your confessor didn’t ask for it, your confession would still be valid. Stating the length of time is a custom which is observed because it is helpful to the confessor (and yourself) to ensure that you make a complete confession and to provide counselling.
So, just be as specific as you need to be to assist your confessor, rather than treat it as a statement in court. Don’t agonise over it, and don’t hold up the confession with… "It’s been 6 days… oh… wait… maybe 7… I’m not sure…I think it was last Saturday… ", etc …
I usually mention 6, 7 or 8 days as “one week”, unless I am confessing a frequently occuring sin, by number, then I may be more specific. eg. “It’s been eight days since my last confession, and I have committed 4 times”, as this gives a better picture for the confessor. (or, I may not… and this is also valid).
For five days, I would always say “five days”, as that is the most simple, natural way to describe that period of time. However, if I had said “one week”, I would leave it there, even if I was still in the confessional.