How many hours into the PPL training do you HAVE to start doing exams?
Many schools want you to pass Air Law before first solo, although this is not a legal requirement.
If by contrast you are only able to afford one flying lesson a fortnight...
... then don't do that, as you'll spend two thirds of each lesson getting back to where you were last time, thereby wasting most of your money. Instead, save up for a few months, then book a batch of lessons at the rate of two or three a week until the money's gone, rinse and repeat.