Use a stick to create a sundial. Now start counting your heartbeats. Very roughly you can expect ~5,000 per hour (80 BPM). Make marks where the shadows lie every ~2500 beats or so. Make sure you're in a resting state when you take the measurements.

Do this over and over on several days and you should be able to start making a good estimate of the planet's day length and seasonal cycles, with a value defined by your resting heart rate.

Now you need to estimate what your heart rate is compared to earth time. Don't go for an exact value, like 83 or 65. Instead try for a range. Maybe you're pretty sure your heart rate is between 74 and 89 BPM. Take your day data, combine it with your estimates, and that will give you a lower and upper bound on the number of days before the portal will re-appear.

Now use the time before your lower bound to prepare - gather food and water, create shelter, see if you can set up some sort of alarm, etc. Then wait and hope.