How to convert it
Divide the price of a term by its number of months. That shows what you pay per month and lets you compare the terms fairly. The pricing page already shows that amount.
When short is smart
If you do not know IPTV yet, or want to see how it works on your TV and your connection first, start with 3 months. You pay a little more per month, but you risk very little.
When long is smart
If you watch all year, 12 months with 3 months free is the best value. You pay once and have nothing to think about for fifteen months.
And the number of screens?
The term is not the only thing that sets the price. Every extra screen costs more, so only choose what you really use at the same time. One screen is enough if you watch together on one TV. Convert this to a monthly amount too, and you quickly see whether a second screen is worth it to you.