Full crawl must be done at least once, after this you can start incremental crawls. We had some SP farms where full crawl takes 4 days...
You can split content to more Content Sources, where you will define starting Site Collections, this is the best way. Also check the SharePoint server usage, it cannot be at 100%, if you have not enough CPU or RAM, you should add more restouces to the server.
Full crawl can take up a hours, days, it depends on:
- size on crawled content
- security - how many there are unique permissions, lots of users, groups, etc
- settings of Search - Performance Level, correctly Content Account, etc
- hardware of your servers
- correct topology
Here is the MS link check it.
If you want more help, just contanct me, I need more information or we can do some TeamViewer session.