Building a custom system is expensive, and maintaining one is more expensive still. Our default advice is almost always: while off-the-shelf software works, use it.
But there are three signals which, when they appear together, mean the cost of not building has passed the cost of building.
First: staff copy data between systems by hand. Anywhere a person is acting as a bridge between two pieces of software, there is a broken process that costs time and manufactures errors.
Second: a process that is your competitive advantage is being bent to fit the software’s limits. If your pricing model or supply chain is something competitors do not have, it should not be squeezed into a generic tool.
Third: licence costs rise in step with your growth and have reached a number comparable to the cost of building.
If only one of the three applies, it is probably not time yet. The cheaper answer is usually integration, not a rewrite.