I change mascaras that i use frequently at around 6 months. Mostly cos I've actually finished them by then or that the last bits have dried up. If you hardly open the tube you can keep it a 'little' bit longer but you might as well change anyways because new and better ones are constantly being released. Don't 'pump' the mascara brush up and down inside the tube. That makes it dry faster. If that happens you can stick the opened mascara tube (without the brush) into the microwave and give it about 15 seconds. That's provided the tube is not expired la

On which brand is better, depends on you. I'm currently loving my collection of Majolica Majorca's mascaras. Good results for the price. Lash gorgeous wing is very lightweight to wear. That's my everyday tube. I didn't have great experiences with Loreal, Revlon or Silkygirl, those have pretty thick formulas that didn't suit my eyelashes. I did however liked Maybeline's Lash Discovery as it was very natural looking but smudged after 6 hours.
The best mascara I've had? DiorShow Iconic and Blackout. You don't even need an eyelash curler for those ^____^