Number of iterations allowed for each range of n :
1.n=10^5- most used constraint in competitve programming , three types of complexity is allowed in these type of questions . fist O(n) ,O(nlogn) and O(nrootn) , you have to use fast io in O(nrootn) complexity.
2.n=10^6- only two types of complexity is allowed here, first O(n) and O(nlogn).
3.n>=10^9- you can use maximum of O(logn) complexity.
4.n=10^3- these kinds of questions requires O(n), O(n^2) or O(n^2logn) complexity.
5.n=10^2- you can use O(n), O(n^2), O(n^2logn), O(n^3) orO(n^3logn) complexity.
6.n<=20 - These question requires exponential complexity. mostly bitmasking questions are given in these ranges of n.
So basically use can use maximum of 10^8 iterations , but if you are using above 10^7 iterations make sure you io method is fast,sometimes simple io doesn't fit in time limit.
and obviously O(1) works for all kind of constraints.
Hope it helps.