i almost completely reinvent my trading strategy every 8-10 months.
I'm not sure that's really optimisation, we're talking about some kind of parameter fitting process I think.
I run a daily optimisation to determine what trades I should do, but the parameters that control the forecasts which dictate those trades are never optimised. So I answered 'never'. The parameters are based on 50 years of data. Having another year of data is irrelevant.
Most people optimise too much and too often. Even HFT guys only optimise about every 3 - 6 months.
A lot of this is to do with people not designing strategies that automatically cope with changes to eg volatility, price levels and so on. If you don't scale your forecasts according to vol estimates, you'd have to reoptimise every time volatility changes.
GAT