How much can this program hold? Can I put 10 years worth of writing in it?
That's a tough question to answer. Are you just using text? Are you adding large MB images to your entries? Do you have limited hard drive space? There are a number of factors but to put things into perspective, writing a few paragraphs a day and adding a few normal-sized images a week, and with a decent amount of hard drive space, it would take many, many years to see any sort of significant slowdown.