Filling out online application form for a tourist L

I am a USA citizen but I live in the UK and have an indefinite leave to remain visa for the uk. I am trying to fill out the tourist application form and I need to enter the expiry date of my stay for the uk as i am applying in a country other than my nationality, but it does not have an expiration... how do I fill it out as the online version will only let me enter a date?