I import some alembic to Houdini. How do I know what frame to start and end?

I don't know if you can. Just move in the timeline until the object starts and stops moving.
It should have the start anywhere between 1 and 1001. 1001 is the shot's first frame in many companies so maybe what you have is following that convention.

You can middele click on the node in the networkview

or if You need it automated:

the values are the actual samples, so if You have motion samples they have a additional step

sanostol
You can middele click on the node in the networkview

or if You need it automated:

import _alembic_hom_extensions as abc
time_range = abc.alembicTimeRange(archive_path)
start_frame = timerange[0]*hou.fps()
end_frame = timerange[1]*hou.fps()

the values are the actual samples, so if You have motion samples they have a additional step

sanostol
You can middele click on the node in the networkview

or if You need it automated:

Thanks 
import _alembic_hom_extensions as abc
time_range = abc.alembicTimeRange(archive_path)
start_frame = timerange[0]*hou.fps()
end_frame = timerange[1]*hou.fps()

the values are the actual samples, so if You have motion samples they have a additional step

One inconvenientce may be that those functions return timesample at 0 for static meshes

Therefore if the real animation range (in frames) is 1001-1100
And if you also have some static meshes, it will return something like 0-1100, simply not the actual animation range