If you are after longevity in storage, leave the mod charged at 3.9 Volt per cell, and top up every three months.
Temperature is important, never ever store above 25 C. If you live where summers are at 35 C, you need to top up every month. But, the storage life of your mod will be about half.
I am forgetful, and have a lot of internal battery mods in my collection.
I don't care of longevity, as I replace the internal batteries, in the mods I value most, after three years.
Therefore, I charge them at 4.2 V (100%) every three months, even if this will shorten battery life somewhat. In this way, I can use them without charging them first.
A recommendation. Beware of standby power consumption of a mod. Some are just excessive, in that case the mod is not fit for long term storage.
Another recommendation, this time about multiple Li-Po packs, especially the 3s. As in the eLeaf 200QC, some DNAs, Dotmod, etc. I keep a special eye on those batteries, as they get out of balance if you look funny at them.
If you want play safe, long term store only single cell mod.
What has behaved best for me, and I vouch for, is the Smok R80, Smok Q-box, the iStick 20W, the newer eLeaf 80 W (it uses a single 5000 mah cell which is 4000 mah), the Kanger k-box. Testing lately the V-Fin, I can't vouch for it yet.
Good luck...