My recipe calls for grilling a pork loin for 25-30 minutes over medium high heat with the lid closed. How do I convert this to oven cooking time?
Cooking meat by time is ill-advised, whether it is on the grill or in the oven.
Instead, you should cook by desired outcome, which is the final internal temperature. Pork is done around 155 F / 68 C, so you should roast it in an oven until it is just a few degrees below the target temperature, as measured by an instant read or probe thermometer inserted into the thickets part of the loin.
With a 400 F / 200 C oven, that is likely to take somewhat less than an hour depending on the size and shape of the particular loin you have, but the internal temperature is what will tell you it is truly done.
Note: you may want to remove it from the oven when it is a few degrees shy of the target temperature, and allow so-called carry over cooking to bring it up the rest of the way as you let it rest. Personally, I like my pork a little more done so I stop roasting when it registers 155 F, and let it carry over a few more degrees.