Hi Clare
Merpati are notorious for changing their flight times so it is always essential to
1. confirm your booking the day prior
2. on the day of departure, if practical, phone and ensure that the flight is currently on time (which is still subject to change
)
Your hotel or losmen will be able to get Merpati's number for you, and even phone and confirm for you.
The middle of the day flights tend to be worse than others. Last time I flew Merpati it was 3 hours late and it was lucky I had allowed several hours in Bali otherwise I would have missed my connecting flight out.
You should arrive at the airport at least 1 hour prior to departure. Checkin closes 30 mins prior but don't ever aim on being on time in Lombok. All you need is a problem with delays you to getting to the airport and you will miss your flight.