If you have the infused oil around for awhile, put it in the fridge or freezer. That will help slow down oxidation (rancidity). If you can keep the jar as full as possible to eliminate the air space, that will also help -- if need be, transfer the oil to a smaller container.
The time it takes for the oil to become rancid depends on a number of factors. One is the choice of oil. Meadowfoam might give you 2+ years. Sunflower would probably be fine for 6 months to a year. The temperature, exposure to light, and any possible organic contamination or water in the oil will also affect the useful life of the infused oil.
Someone recently put some petals (not sure if they were calendula) on the top of her soap and the petals became moldy. I'm not saying this will always happen, but it's an issue to keep in mind, especially in a humid, warm environment.