Depending on what you're doing I would stick to 140 to 150 bpm and do no more than 30 mins and an intermediate pace.... mix it up with HIIT you'll be fine....just be consisitent
i stay consistently between 130-140 at max. I stay on for minimum 45 min. For fat loss i'll do fasted cardio right in the am or post workout after my body is depleted of glycogen