In my opinion, this is a classic "How long is a piece of string" question...and frankly it's impossible for someone else to answer.
Why? Because every single Panerai 'collector' has his or her own priorities in terms of what they like or are searching for. If you look at my signature you'll see a number of watches gone from my current collection - why did I buy them only to get rid of them? Simply because, for me at least, it's evolutive in the sense that I purchased what I 'thought' I liked until I finally settled on the style/shape/movement that I'm happiest with.
I have
never considered the future resale value or whatever when I've purchased - I've mentioned many times on this forum that I buy a watch for ME, not to 'flip' or with the hope that someday it'll increase in value and I'll make a killing. I might add that I have sadly never made a profit when I have sold a watch - ever!
The watches I own are all very 'particular' to me, either they have a certain serial number, or a particularity which makes them 'special' to me.
In addition, your own personal finances must decree whether or not you must sell to buy again. I am selling two because, to me, they're not 'special' enough to keep. Sure, I could do with the money, and will without doubt buy others, so I'm not a reference in this
I seriously think that there is only one person who can answer your question...sorry if this isn't much use to you