At any time before the end of the tax year to take effect for that tax year by means of form 149.
Likewise, one is excluded from the Regime if there is non-compliance with any of the requirements. Such non-compliance must be reported to the AEAT within 1 month. The change of the employer by which initially came to Spain for another employer, or the signature of a new contract with the employer, does not determine the expulsion from the special regime (DGT CV 28-3-16; CV 4-5-17), being able to remaning a short period of time in a situation of unemployment or inactivity between the cessation of the labor or administrator relationship that motivated, in a real and effective way, the displacement and beginning of the new labor or administrator relationship (DGT CV 6-717.
If you renounce or are excluded from the Regime, you will not be able to opt for its application again.