R help - Dec 2010 - Determining period of a discrete dataset

I have a dataset that goes like: dataset t___|x___
0    |x1
1    |x2
2    |0
3    |0
4    |0
5    |0
6    |x3
7    |0
8    |0
9    |0
10  |x4

and so on. I wish to detect the periodicity of occurrences. t is in
seconds and x are arbitrary, whose magnitude i am not interested in. I
just wish to get a best guess for the period of mere occurrences of x.
Tried using spec.pgram(dataset) but the periods I get are too small -
of the order of 1e-5. What am I doing wrong?