There is no limit.
That is the whole legal answer. The tax code caps how much you can contribute to an IRA each year, and it caps nothing about what percentage of that account holds gold.
You could roll over $900,000 tomorrow and put every dollar into bullion. No form objects. No custodian is required to stop you. The IRS does not distinguish between an IRA holding one hundred percent metal and one holding one hundred percent index funds.
Which is precisely why this question deserves more than the legal answer.
"Can I" and "should I" have very different answers here, and the entire gold marketing industry is built in the gap between them.
So let's do both. What the rules actually restrict, and what the sensible number looks like.