[NM]

The Visa Waiver Program permits entry for up to 90 days for the purpose of tourism or business. So long as you meet those requirements there should not be a problem. I have had multiple entries in a 3 months period (at one time spending 2 our of every 4 weeks in the USA), and have never been asked for proof of planned departure date.

They are very strict on you not seeking employment while in the USA. You are likely to be asked about your line of business or company you work for, so be prepared to answer those questions.

Do your research into the VWP requirements to see if visiting a girlfriend is a legitimate entry activity. If it comes up, then make it clear you are not seeking residency while you are there as that may be a breach of the VWP entry criteria.