If you stay for more than 90 days out of any 180 the spanish authorities will deem you to be a resident. You then have to fill in a form 720 where you declare all your foreign assets over £50000 to the spanish authorities ie.
Foreign bank accounts, regardless of the currency
Financial assets, like stocks, mutual funds, ETF’s, securities, insurance policies, endowments, pension Funds, and similar investments
Foreign real estate properties or any other interest therein.
As you are deemed to be a spanish resident any foreign income (eg pension) will be taxed here.
I personally won't be exceeding the 90 day limits.
it's worth googling this, there are some horror stories.