Sorry for the confusion SWILT. What I meant to say was that taxation treaties between countries help prevent individuals from being double taxed. So whatever time limit the tax treaty has, you are somewhat protected from paying taxes to the US and the country you live in as a US citizen. For Germany, the tax treaty with the US was or is 10 years. So in our case, if we had lived in German for 10 years, both the US and Germany could have taxed us on our income. The government view is that after 10 years, we have opted to be permanent residents of Germany and thus should pay taxes as "permanent" residents. The foreign tax credits would disappear and double taxation could take place.
I don't recall exactly how long our tax prepare said the UK/US tax treaty can be applied to a tax payer. I did read in my info pamphlet that the UK will consider you "domiciled" if you live 17 out the past 20 years in the UK.
With regard to CA, it is very hard to break residency. CA knows that many individuals have second homes in the state so they make it hard live in the state and avoid the high state income tax. Read Publication 1031. You will find info on how residency is determined. In our case, we own homes in CA and most of our financial accounts are in CA. So when we lived abroad, we had to continue filing CA tax returns as non-residents. But in order to qualify as a non-resident in our case we could only spend less than 45 days a year in CA. If we spent more, we had to pay taxes for those extra days on ALL our income WORLDWIDE and not just CA sourced. It is hard to break ties to CA. Once your wife stops filing a CA tax return, the state may ask why and as in our case may want a copy of her foreign visa and a copy of every page in her passport to see her travel.
Being a US citizen and living abroad, can have many tax implications. Thus, we have always worked with a large accounting firm to know the implications. For example, assets in the US inherited by non-US citizens or Non-Green card holders have different tax exemptions than those for US citizens and Green card holders.
https://www.ftb.ca.gov/forms/2015/15_1031.pdf