How long does it take for a dog to decompose?

My mother owns large (bush land) acreage property and unfortunately our family pet,14 Year old Border Collie past away of what we are assuming was natural causes on the 1/01/17 while she was out shopping.

We are devastated and my mum feels guilty for leaving her alone, she decided to bury her under a tree in a sizable hole her partner dug, mum wanted to wrap her in a blanket or place her in a box but her partner said it would be fine to bury her as is.

Mum has not been able to sleep as she fears wild animals will smell her decomposition and she will wake up to find parts of her scattered around the property.

How long does it usually take for the body to stop smelling, I would like to put my mums mind at ease.

Thank you.