Generally Eleaf products, the vendor will a lot of times support a 30day warranty after date of purchase, after which, a device is under the manufacturer 90day warranty, which would be Eleaf you would need to contact.
Most batteries though are rated for 150 to 200 full charges, mod shutoff voltage we will say 3.0v to 4.2v, half charges, from say 3.7v to 4.2v will net about 250 to 400 recharges depending on how badly you abuse them. Eleaf iStick mods /w internal batteries with a lot of abuse, running full power and recharging a couple times every 1 to 2 days takes notches out of the mod battery life. The fact you got about 4months out of the mod, you got decent life, thus why I myself I do have a couple mods with internal batteries I barely break 15watts on them for out and about, after that, 90% of my mods use replaceable batteries, battery gives out the mod is still good, just put in another battery. I would be looking at a new purchase, maybe a Kanger Sub-box/Top-Box Mini, Sigelei 75w, Wismec just released a new 75w mod that is pretty.