We've had the storage heaters on to some degree since the last week of October this year, when outside it was dropping colder than 6C in Hampshire and inside it wasn't getting much beyond 16C in the back room.
We could have waited a bit longer if we only had to be concerned about us being comfy, as it's a bit warmer in the lounge from the TV and computer plus the fish tank's lights/filter.
But my three active fish tanks in the back room don't run heaters despite being occupied by "tropical" fish and they don't run lighting either because until the lounge tank they don't contain live plants. Their water was approx 18C, but I wouldn't want it to drop much below that for African and Asian species I have (Weather Loaches and Mountain Minnows would be fine, but I keep neither these days).
The back room only has a small storage heater, so that's been on maximum charge each night now for ~8 weeks.
Our flat used to get bitterly cold in the winter, especially when their was an easterly or north easterly wind. We then discovered our rental flat had paper-thin loft insulation and no cavity insulation, got both sorted on grants and it saved us ~£500 each winter!
On the flip side, my mum's bungalow just up from the North Wales coast very close to the place that shall not be named, doesn't get much direct sun from the south even in mid summer. With thick walls, her place rarely gets above 15C inside in mid summer, while in the regular heatwaves of recent years our lounge has hit 30C or more.