It is valuable when thinking about distant galaxies to be calibrated on
the timescales
involved. Based on a reasonable set of numbers, i.e., an open cosmology
with H0 = 65 and q0 = 0.05,
t0 = 15 Gyr, with the lookback time at z ~ 1
is about 50% of t0,
while the lookback time to z = 3 is about 11 Gyr. Or, in a way
that is better for thinking about
formation timescales, the time since recombination to z = 10 is
about 1.4 Gyr, to z = 5 is about 2.6 Gyr
and to z = 3 is about 3.9 Gyr. The timescales grow somewhat
longer with a lambda-universe,
with 
= 0.7 and
m = 0.3, and dramatically
shorter for an Einstein-de
Sitter m = 1
cosmology (giving a rather unrealistically
short timescale for galaxy development by z ~ 5 of < 1 Gyr!).