It also depends on the type of battery in the phone.
Here are a few tips on how to prolong the battery life.
For NiCd (Ni Cad) discharge batteries that are in regular use to almost dead every 1–3 months to prevent battery capacity memory. This also applies to lots of partial charges (meaning not to full capacity before charging again)
For Lithium ion keep battery cool. Don’t leave it in hot places. BEst to operate when still at least 25% charge left and limit the use of ultra-fast charging and high loads (most Li-ion). This can damage the battery in the long run.