Killing a titan is not an action performed by a single player, but by an entire alliance. Titan kills objective can ruin alliances eco-system and moreover could be unfair for active players that can’t easly kill 35/50 titans inside their own ally.
So I would suggest to replace with titan attacks: 150 total (3 titan attacks * 50 days)
tier 1: 5 atacks
tier 2: 15 atacks
tier 3: 30 attacks
tier 4: 40 attacks
tier 5: 50 attacks
(total: 140 attacks). I think that 3 attack for every day with a margin of 10 days would be fair. 150 attacks total could be acceptable (but not so suggested). More than 150 would be unfair.