A fixed deposit can be made for a large number of time periods.
The choice for an investor with a particular institution depends on the options being offered by that institution.
Usually a bank will give more choices thus you can make a fixed deposit here for say 3 months if you have that time horizon or even 30 months if that is what you want.
Other financial institutions and companies might have a specific few choices available like 1 year, 2 year or 3 year deposits.
An investor should not assume that the time period available with every institution will be unlimited and hence should work towards matching their needs and the choice available.