The -1/-1 counters remain on the creature indefinitely. They’re not removed if the creature regenerates or the turn ends.

Infect’s effect applies to any damage, not just combat damage.

Damage from a source with infect affects planeswalkers normally.

“Protection from artifacts” means the following: -- Tel-Jilad Fallen can't be blocked by artifact creatures. -- Tel-Jilad Fallen can't be equipped. It also can't be enchanted by Auras that have somehow become artifacts in addition to enchantments. -- Tel-Jilad Fallen can't be targeted by abilities from artifact sources. -- All damage that would be dealt to Tel-Jilad Fallen by artifact sources is prevented.

Damage from a source with infect is damage in all respects. If the source with infect also has lifelink, damage dealt by that source also causes its controller to gain that much life. Damage from a source with infect can be prevented or redirected. Abilities that trigger on damage being dealt will trigger if a source with infect deals damage, if appropriate.

If damage from a source with infect that would be dealt to a player is prevented, that player doesn’t get poison counters. If damage from a source with infect that would be dealt to a creature is prevented, that creature doesn’t get -1/-1 counters.

A player who has ten or more poison counters loses the game. This is a state-based action.