How long do I have before I need to enter the UK?

I received an endorsement this week (!) but am confused about stage 2. On my application for stage 2, do I need to write down that I'll be arriving exactly 3 months from the date of submitting the application? Or can I choose a date further out?

My husband will need to find a job between my submitting and us entering. Not sure that can happen in 3 months. Also, what if stage 2 takes longer than 3 months?

TIA!