Basically, endless.
But (!) only non-excessive shares will give you upvotes (and income) from steembasicincome.
Right now the amount of shares that will yield a return is 800. (Eight pools with a maximum of 100 shares each.)
After the transition it should be 1,000 shares. (Eight pools with a maximum of 125 shares each.)

You got one share in pool 5 (@sbi5), so to max out all pools you need to sponsor 799 shares or be sponsored 799 times. (999 times after the transistion.)

After that: You would need to wait for new pools to open. I assume that every excessive share over 800 (1000 after the transistion) will automatically go into the new pools. But in the meantime those excessive shares won't give income, but they are going to help steembasicincome.