Generally signals are placed around 1000-1200 meters between each other. If the distance is greater than 2 km, such signal should have its own separated distant signal again around 1000 meters ahead.
The signals that are placed in the regular 1 km interval are generally the automatic block signals, they have three lamps and white mast (pole). Classic main signals where the distance is more varying have red and white masts and they are pre-signalled by another main signal, by standalone distant signal (the one with large X in square desk), or by last post of automatic block (still white mast, but with additional desk with black-and-white circular "target" sign under the lamps).
If the distance is insufficient for braking, usually you are signalled as such in advance – in automatic block, the post before one showing caution is showing advance caution (flashing green), when in stations with multiple main signals in less than 1 km, you should be slowed to 40 km/h by the last passable signal.
I think it happened to me once that i got red in like 500 meters (emergency brake still worked with few dozen meters to spare), but that might have been a dispatcher revoking the signal. Otherwise I drive without HUD quite okay-ish, speeds are worse (they are also announced rather consistently in advance, but sometimes with really tiny signs).