The half life for levothyroxine allows some room for error, don't skip a dose, just take your next one as scheduled - for me I like taking mine either first thing in the morning ( if I wake up early and will be going back to bed ) but if my sleep is doing better and I'm not waking up multiple times I take it right before bed

Also the answer about how long you can go without depends; have you been taking it long enough consistently for more than a month? If yes, if you happen to miss a day I wouldn't worry about it but if you're not consistent then it doesn't have a chance to build up in your system and it won't help you

I use medication reminder on my phone I found the app in the App Store and add your medical information , doctors and family contacts for emergencies ( health app)