M.R.
Our son transitioned around 14-15 months and I was terrified! I wasn't ready for "only one nap", but it became the best thing ever! Besides, it isn't about what *we* want and what is convenient for *us* but what is best for the child.
How did I know it was time? The morning nap was great but the afternoon nap was awful. He wouldn't sleep, he'd only sleep for a short period of time, etc.
How did we transition? When the AM naptime rolled around, we switched up what we were doing to distract our typical schedule. Usually it involved leaving the house and doing something really fun. At this early stage, he would only be able to stay awake until about 11:30...but he would sleep til 3:00! He also showed signs of wanting to go to bed earlier than usual (6:00 - 6:15 as opposed to his typical 6:30), so when the telltale signs appeared he went to bed. Eventually he stayed awake longer and longer until the naptime was set around 12:30 - 1:00. He will sleep at least until 3:30 each day.
I was so used to the 'two nap' routine and dreaded only having 'one nap' but it was the best transition ever.