The length of time a star lives for is dependent on mass...the more massive a star, the hotter and brighter it is, the shorter its lifespan. Metalicity will effect how a star evolves but not to the same extent as the initial mass of a star. Metalicity effects the opacity of the gases, temperature and other characteristics. Usually, stars with low metalicities have a higher surface temperature for a given spectral type than "normal" stars (those with similar metalicity to the Sun). For example, a G2V class star in a glob', because of its low metalicity, will have a surface temp of around 6100-6200K, as compared to the Sun's 5800K. Funnily enough, the two stars will have roughly the same luminosity because the glob star's low metalicity effects its size, which is slightly smaller than the Sun (around .85-.9 solar radii).
In general, the lifespan of most stars is equivalent to 1/M^2.5...so knowing the Sun has a lifetime of about 10billion years, you can get an approximate age for most stars by multiplying the answer you get by the Sun's lifespan, thereby giving you the star's lifespan. For example, take Sirius, it's mass is 2 times the Sun's. So plug that into the equation and you get... 1/2^2.5 (x 10^10) = 1.8 billion years. For a typical 5 solar mass star (B0) you get 179 million years. For you typical K class dwarf (K0-3, .85M), you get 15 billion years.
Where metalicity does effect the initial mass of a star is during their formation. The less metals are in a gas cloud, the more transparent its gases are, the hotter and higher its mass will be for any given star that forms out of it. There's a property of a protostellar gas cloud called the Jeans Mass, which is very dependent on mass temperature and metalicity, amongst other factors. Very early star formation, which basically occured in almost pure Hydrogen and Helium, precluded the formation of small (smaller than 10 or so solar masses) stars as the clouds couldn't become dense enough and divide up into smaller units. These pure H/He protostellar clouds had high Jeans Masses, so the stars that formed out of them were large and very bright, as a consequence. Because of the absence of metals in the gases, many of these stars (called Population III stars) grew to extreme sizes...250-1000 solar masses. So you can imagine how bright some of these stars were. Probably upto 100-500 million times brighter than the Sun!!!...especially for those stars of early spectral type (O to A class). Some of the biggest and brightest probably burnt themselves out in less than a million years. A typical O class Pop III star, because of the absence of metals, would've had a surface temp of around 70-80,000K and was most likely 200-400 solar radii in size. Also, the absence of metals effected the end products of nuclear burning in their cores. Because the gases in these stars were nowhere near as opaque as in later stars, hence the density of the gases was substantially less in the core regions and nuclear burning could only get as far as Oxygen before the stars went hyper/supernova. Metals such as Sulphur, Silicon, Magnesium and Iron came from later stars, once the metalicity of the ISM reached a certain level to be able to support the growth of higher metal stars.
I hope that's helped answer some of your questions
