postgresql reindex frequency and monitoring

What's up guys. I have a couple of tables where i inserting much of data (and almost never deleting). Periodically i see that insert gains ExclusiveLock on index of table. So i guess that sometimes insert triggers reindex of table. So i wonder how often it happens and how can i see in logs how much time it took and how often it is triggered? Maybe some blogs/docs on it ? thanks in advance.