There is no standard answer to this question. Founders need to have a private and honest conversation about what split among them (or them an any potential founder to be brought in early). The default is usually an even split, but that split is best if it is the result of a reasoned decision-making process about the relative value brought into the enterprise, the future role expected, etc. Keep in mind the usual need to reserve some equity to incentivize early hires.
It is also common for founders to have vesting imposed on their shares, so that if a founder leaves the employ of the company the company can repurchase their shares. More information about vesting can be found here.