I'm about to graduate, and received an offer from a company I'm interested in. How long can I wait/drag out the offer in case another better opportunity comes along?

Ask. Ask. Ask.

Don't be afraid of calling HR to ask when is the latest you should get back to them!

If anything, they will be happy to provide a date so both parties won't be kept waiting unnecessarily.

Always ask the HR when is the latest you should be responding to the offer. If you’re interested in this company, it is unlikely that you’ll look away from it. And since you’re willing to consider another company’s offer, which is currently still an imaginary, you might not be suitable for this company? Because you’re just looking out for “who can offer me the best”. Even if you pick up this offer now, soon you’ll choose to leave because you’re not thinking of the intangible gains from the company and not even what you can contribute to the company.