A constant half-life is necessarily an exponential decrease (linear curve on a logarithmic plot). So, yes, if the half-life is 6 hours without qualification, then the quarter life will be 12 hours.
Though, in real-life you get things like enzyme saturation, depletion of chemicals consumed in the metabolism of the chemical, etc. Real world pharmacokinetics are probably not exactly constant half-lives.
For example, IIRC, the average American adult male saturates his alcohol dehydrogenase at about 3/4 shot of 80 proof liquor per hour. Below saturation, I presume ethanol has something close to a constant (and short) half-life, but that exponential decrease is only observed at alcohol concentrations where the effects of alcohol aren't very obvious. So, for most practical purposes, the biological half-life of ethanol doesn't apply and its metabolism is better approximated as a constant rate process.
Though, in real-life you get things like enzyme saturation, depletion of chemicals consumed in the metabolism of the chemical, etc. Real world pharmacokinetics are probably not exactly constant half-lives.
For example, IIRC, the average American adult male saturates his alcohol dehydrogenase at about 3/4 shot of 80 proof liquor per hour. Below saturation, I presume ethanol has something close to a constant (and short) half-life, but that exponential decrease is only observed at alcohol concentrations where the effects of alcohol aren't very obvious. So, for most practical purposes, the biological half-life of ethanol doesn't apply and its metabolism is better approximated as a constant rate process.