What will happen with items I put into someone's inventory during Pickpocket?
Is there a limit things I can put into someone's inventory during Pickpocket attempt?
I haven't tried testing reverse-pickpocketing until reaching the maximum limit, but in theory, it should be limited by an NPC's weight limit (which is dictated by their stamina, which is affected by level, race, equipped items, class, specified carry weight limit, game bugs, among other possible factors). This is similar to how followers will have a limit to the weight of items they can carry. Related question: How much can my follower carry?
NPCs can be used as containers either via pickpocketing or trading with followers, but have a weight limit as to how much they can carry.
Source: UESP wiki, "Containers" article
And another question - what will happen later with items I put into someone inventory?
Depends on the item. If it's a poison...
If you have the Poisoned perk activated, you can reverse-pickpocket a paralysis potion into the target's inventory, which the target will subsequently drink. The character will become stiff and fall to the ground for the duration of the effect, during which time you will be unable to interact with them (save for attacking of course). As the effect wears off and the character starts to get up, immediately pickpocket them again. Though the % probability displayed will be the same, the game will treat it as a corpse looting instead of a pickpocket, which means the actual success rate will be 100% for all items and all gold amounts and you won't run any risk of incurring a bounty. This is especially useful for training skills at higher levels and getting all your money back.
If you fail, the target will detect you and you will receive a bounty. Even if successful, there is also a chance that the target will hire thugs to kill you.
After investing in the Poisoned perk you can reverse-pickpocket poisons onto targets to damage them. Giving multiple poisons to a target will make them "take" the poisons until they are either all gone or the target dies. Remaining poisons will be left on the corpse and are retrievable.
Source: UESP wiki, "Pickpocket" article:
If it's a stolen item...
The stolen flag can be cleared from items by selling the item to a fence and then buying it back. If you have the Merchant perk from the Speech perk tree reverse-pickpocketing the item onto a merchant and then buying it back also clears the flag.
Source: UESP wiki, "Crime - Stolen Items" article:
what will happen later with items I put into someone inventory? Will these items be wiped out or maybe carried by this person all the time?
I know from lots of playing and lots of pick pocketing that items placed into a normal NPC's inventory tend to stay there while you are in that town/city etc. but if you leave and don't come back for several days they will be gone. I have also run into the reverse where I will steal something from an NPC like boots, clothes etc. and those items will re-appear in their inventory if I leave and return a couple weeks later.
Note: I do not know if this is caused by a time limit for pick pocketing OR if there is rather a limit on total number of NPCs pick-pocketed, because I pretty much pick pocketed half of skyrim!