Answer: You will need to bring it indoors before the weather turns cold; your goal is to minimize the temperature shock when it is moved. Give it as much sun as possible indoors, along with a cool room temperature and ample humidity.
In spring, take it back out when nights are reliably in the mid fifties again. Or, you can begin setting it out earlier on nice days in the fifties and bring it into a garage or similar protected area at night until it is safe to leave it outside permanently for the summer. In spring, gradually acclimate it to the changing light level when moving it over a period of about two weeks, slowly increasing the direct sun it receives to avoid "sun burn" on the foliage.
Lemons generally ripen in winter, your crop should continue to thrive as long as the plant receives ample light and is not overheated while indoors. If it is stressed or even if conditions are just not quite right indoors it may drop some or all of the fruit.
Citrus generally does well even when relatively potbound. If you need to repot, do so in the spring. A good quality, general purpose slow release or water soluble fertilizer such as 10-10-10 with micronutrients should be adequate, feed spring through fall.
Here is some general container citrus care information you may find helpful. You might need to cut and paste the complete url into your browser to make it work correctly.
Good luck with your Lisbon Lemon!
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