You are only allowed to hold 30,000 bonds, and the way the pot of money is divided up is designed so that for each 30,000 of bonds in existance there will be 1 win each month..... so:
If you hold 30,000 bonds you should win each month
If you hold 10,000 bonds you should win every 3 months
If you hold 5,000 bonds you should win every 6 months
If you hold 1,000 bonds you should win every 30 months
The draws are random (good old ERNIE), so just because you should win doesn't mean you will, but also maybe you should wait 30 months, but will win the jackpot on the first draw.
The minimum win is £50, and all winnings are Gross (i.e. tax free)
:D