Without a SIM it doesn't use much power at all. Even an Android phone will last a month on standby if you just leave it on the table with all connectivity disabled.
I remember back in 1998 when Philips introduced GSM phones with 300+ hours standby battery life (with a bigger battery). Until then it wasn't uncommon for phones to last 8 to 20 hours. Then others figured out the same, but long standby was never practical unless you really just leave a phone on a table.