Heating times in microwaves depend on volume of milk, temp of milk (whether its been in the fridge, or has just been made up) and microwave wattage. Generally for around 180 - 200 ml try heating for 30 secs. Shake bottle well, test temp and if more time is needed try 10 second bursts. Always test before giving to bub, and if using someone elses microwave, proceed with caution as everyones seems to heat differently.
Bottle warmers to me are not generally great investments. If you check boxes for heating times required you will find most need around 8-10 mins to heat 200ml (i think). Fine if you have a patient baby, i do not. You are just as well off heating with hot water.
Hope this helps!
Rachel, SA, mum to Ella 11mths