What's the impact of archive_command failing?

I have postgres setup for streaming replication and my slave box went down.

My question is, how long can that box stay down before it causes a material impact on the master?

The archive_command that I use will not archive logs while the slave is down.

I know the obvious problems:
 * you're not archiving, so you lose all of the recovery related benefits if you should crash
 * could run out of disk space in pg_xlog
 * your slave could get out of sync

What i'm concerned with is something like, if the master is unable to archive eventually
it will stop writing WALs Or something impacting the performance of the database.
or anything along those lines.

Thanks