I need to keep track of calls to clients and the amount of time of each call. Can the call log feature keep for 6 months

It logs the last 100 calls, time doesn't matter. If you need to log more, check on your carriers website. Many of them will keep a longer list of calls.

Sorry no, the call log limit is 100 incoming/outgoing calls.

If you need older calls parameters, you can contact your cellular carrier and ask them for a detailed calls list.

And/or, if you'd like, you can submit your request/suggestion to Apple directly, using this form:--> Apple – Product Feedback - iPhone

Regards

Giulio