Durations of Time

Ago : hace + a period of time
Se fue hace quince días. He left fifteen days ago.

Since / For : desde hace + a period of time (careful with verb tenses here: present tense in Spanish, present perfect or present perfect continuous in English)
No vienes a verme desde hace un mes. You haven't come to see me for a month.

In : dentro de + a period of time (time at which something will happen)
Te llamo dentro de media hora. I'll call you in a half hour.

---