This sounds about right. It's not quite correct to say that 15 held for 30 years has the same odds as 15 x 30 held for one year, but it's a rough approximation. Throwing that number into my calculator tells me that you had a 51% chance of winning nothing, a 34% chance of winning a single €50 prize, a 12% chance of winning 2 x €50, and 3% distributed among all other possibilities. You'd have to hold them for 100 years before your chance of winning something went to 90%. As has been saidI have decided to start tidying up some of my odd purchases of Prize Bonds over the years and to encash these.

My first batch being sent off today were purchased in 1985. A lot of 15. I have never won any prize from any of this batch, even though a rough calculation would show that they would have participated in about 24,000 draws during that time. So a nil return for my €95 investment in just over 30 years.

*ad nauseam*on this thread, a low number of prize bonds really makes no sense as an investment, because the time horizon in which you are likely to achieve the average return is too long for most people. (And recently, the average return has become not worth it anyway).