Social Contributions in Spain (Tarifa Plana)

I don't want to spend hours on this but I had a quick look and this info, which I got from, infoautonomos dot eleconomista dot es seems to be up to date and to tie in with the government site at seg hyphen social dot es which is where the info will be right.

This is the basic scheme

Primeros 12 meses: en 2018, 50 euros justos de la cuota mínima por contingencias comunes o un 80% de bonificación en caso de cotizar por bases superiores a la mínima establecida (919,80 euros)
Meses 12 al 18: 50% de reducción durante el segundo semestre, que se quedaría en 2017 en 137,97 euros.
Meses 18 al 24: 30% de reducción durante el siguiente semestre, que se quedaría con las bases y tipos de cotización de 2017 en 192,79 euros.

But there is an additional paragraph that says

Las mujeres menores de 35 y los hombres menores de 30 tienen derecho a una reducción sobre la cuota de contingencias comunes durante los 12 meses siguientes a la finalización de la tarifa plana, fecha de efectos del alta, equivalente al 30% resultante de aplicar el tipo mínimo vigente (29,80%) a la base mínima de cotización de este régimen.

En estos supuestos la duración total de la tarifa plana será de 36 meses ininterrumpidos. Tendrá una duración de 30 meses en total, ininterrumpidos.

Which ties in with what you have heard that women under 35 have the right to a twelve month extension and that, provided you apply for the tarifa plana as you subscribe to the autónomo/self employed scheme you get 36 months on the reduced scheme though it looks as though that extra 12 months comes in at the 192,79€ rate.

As I said that was with a quick look and I don't want to have to wade through lots of turgid websites trying to find the info.

If you are in Spain any asesor will be able to tell you the ins and outs of this. If you're not you could still find an English speaking one and make the phone call from wherever you are.