Two to six weeks for the cycle is the norm. About 10 days for the NH3
to peak and then drop.
What did you use to start your cycle?
Any Live Rock?
Are you testing your water?
"Note the first 10 days on the above graph. Ammonia begin a rapid rise up to lethal levels and then drops dramatically to close to a zero level. The replication of the bacteria determine this rise and fall. Since they replicate geometrically, the hypothetical first "bug" becomes 2 in 8 hours, then 4 (16 hours) then 16 (24 hours) the 32 (32 hours) then 64 (40 hours) and so on..... It takes about ten days in relatively ideal conditions for the bacteria to replicate to a population where all the ammonia produced within the aquarium is immediately reduced to nitrite. What is shown on the graph is the sudden drop of ammonia concentration slightly after the 10 day maximum."