While the reverse hyper is a great machine and great exercise, it is not the be all, end all in lower back strength. You can get strong without having ever done a single rep.
There are plenty of deadlift variations that will strengthen your lower back. Regular back extensions, both flat and 45 degree, do wonders as well. And don't forget that heavy squatting will also hit your back.
Now if you are doing reverse hypers, great; even if they are makeshift. Start with 3-4 sets of 8-12 reps and work from there. Don't get wrapped up in some magical set/rep scheme. Experiment and do what works for you and get stronger. Be instinctual and listen to your body.
My body says Ding-Dongs are yummy,