When do you switch indoor?

For people living in areas with cold snowy winters. When do you switch to ride indoor on a trainer?

Indoor training is very boring and hard for me, so I want to stay outside as much as possible. But I don't want to ruin my bike (salt, rust) and also want to avoid getting ultra expensive clothing.

I'm considering riding until mid/end of December and then switching inside until April. Please share your plans/experience.