Configured streaming replication, using pg_basebackup command.

Few questions related to same

1) Replication was configured but for some reason standby server rebooted and postgres was stopped mode more than months. its has repo database so not much in standby, after starting postgres instance replication was not working, is there time period replication needs to started

2) Reconfigured replication using pg_basebackup there was difference in pg_xlog logs between primary and standby and also when checked with pg_database_size (disk space size) it was taking time to catch up.

3) how to check the lag time between primary and standby any query

4) i m using pg_stat_replication functions to check the status is that correct ?

Please suggest