Ideally woman's temperature will be slightly elevated for about 14 days (as long as luteal phase lasts), and if you notice your basal body temperature to be elevated less than 12 days that means you should talk to your doctor. Basal body temperature can also drop during luteal phase, and the drop is perfectly normal, and if it returns back, this does not mean the cycle is destroyed. This is because some women have a drop because of secondary estrogen surge. However, this is not the case, if the temperature drops and does not return. What does it actually mean slightly elevated? Average basal body temperature is between 97.0-97.7 before ovulation, and approximately 97.7-99.0 after ovulation, which means in the luteal phase. So we are talking about approximately 0.7 degree jump in average. In ideal scenario woman's temperature will jump for more than 0.5 degrees in the follicular phase, and what is even more important it will stay up and above the coverline during luteal phase.
If the bounce of your basal body temperature is higher than described above (higher/lower than approximately 0.5 degrees), first thing you should do, is to check the health of your thyroid gland. A relatively often a cause of infertility is hypothyroid (thyroid is under active), and usually signifies lower than average basal body temperatures, while hyperthyroid (over active thyroid) signifies basal body temperatures that are too high.