When the ammonia drops to zero in 24 hours, you have graduated to Phase two of the nitrogen cycle - nitrite spike.
Generally, it is suggested that you drop your ammonia dosing to 2ppm daily, to keep the ammonia bacteria colony healthy, but also to keep the nitrite levels from jumping to ridiculous levels. If the nitrite gets too high, it can either stall the cycle or promote the wrong bacteria...
Keeping the ammonia dose small, will keep the A-bacs (ammonia processing bacteria) happy, while not overwhelming the N-bacs (nitrite processing bacteria).
I'd suggest that if the nitrites remain off the chart for another 5-7 days, doing a water change would be helpful. This will help you get a handle on exactly how high they are as well. (Another option for determining the actual nitrite level is to fill the test tube with 1 mL of tank water, and 4 mL of tap water. Add the test solution as normal and read the level. Multiple your result by 5 to get the actual value. If this is STILL off the chart, I'd suggest a water change immediately, because 25 ppm is just way too high! And there is no benefit to waiting that out.)