Is it generally accepted to take months off (unpaid) during a PhD?
This is almost completely dependent on two things: 


Your departmental policies and program structure
Your advisor


In the US, most programs (all of the ones I'm personally aware of) work on a 9-month academic year schedule, so there's generally 2-3 months a year left over. This is a time that is variously spent on research, conferences, teaching appointments, or internships, so you generally want to be on the same page as your advisor (and department) on your plans.

Not all doctoral programs run this way, though, so if this is important to you, you should ask current students, potential advisors, and administrators in prospective programs how this generally goes.

As to affordability, that's really up to you in how cheap you want to (and are able to) live, choosing programs that are in affordable areas, and availability/pay of internships in your field.