I am from the Dominican Republic, just came back from Italy on December 10, 2017. I was granted 45 days. I would like to know if I can ask for another visa to Schengen now on January 29th, to fly over there in April to see my boyfriend. Is it possible? Or is there a time frame to ask for it?
Officially there is no time frame, you could ask for a new visa as soon as the old one expires (or maybe even sooner)...
When checking a visa, an Immigration Officer will have to evaluate how likely is that you would try to remain in the host country. Having complied by the old visa is good, but not enough; because:
If you may stay away from your country for long, it may show that you do not have the kind of links to it (work, family, relationships...) that would force you to return.
Staying too much in the host country may lead to you creating that kind of links in it.
Additionally, the data that you provided in your application must check up. For example, if you claimed that you have a full time job, spending away 3 or 4 months a year is going to raise questions.
That said, I am pretty sure that one visit that ended in December followed by another in April are not going to cause any issues. Just keep the above in mind to try to keep other visits spaced (or make them shorter).