I'm going to introduce some of the subjects that you require to know before you can understand deep learning. My list is not exhaustive, of course. But it should cover more than $60\%$ of the math you need to know. I repeat again that this list is supposed to be minimal and once you start learning these topics, you will realize that you need to understand a bit of other things too. But these ones are the core subjects that you should know.
Calculus (estimated time: 3 months - 6 months)
You need to know calculus well. Most importantly, you need to understand multivariable calculus well because in most problems in machine learning (and neural networks), we're dealing with functions that have many inputs. So, I would say that you need to spend a good amount of time learning about scalar functions, vector functions, gradient, the Hessian matrix, potential functions, Lagrange multipliers, Laplacian, line integrals, Stokes' theorems, etc.
Probability theory (estimated time: 3 months - 6 months)
Computers are wired to deal with true or false values. However, humans think differently. The thing that enables us to teach computers to imitate how we make decisions is probability theory. You need a basic course in probability theory. At the end, you should understand things like a probability space, an event, probability mass/density functions, conditional probability, expectation, variance, higher order moments, characteristic function, the central limit theorem, Bayesian inference, etc. The fun starts after you understand Bayesian thinking. But that's just the beginning. Then you should try to understand harder stuff like Markov chains, stochastic processes, MCMC methods, etc.
Linear algebra (estimated time: 3 months - 6 months)
Linear algebra is probably the most powerful subject in mathematics. You can never learn enough linear algebra. In most cases, dealing with functions is difficult. Nonlinear functions are complicated and they can have very weird behavior in general. However, some functions (which turns out to be almost all of the functions that we're interested in) happen to be well approximated by linear functions. This approximation is done in calculus. The derivative of a function is its linear approximation. Also, linear algebra helps us convert our problem to a language that can be implemented easily in computers. Computers know how to manipulate numbers, vectors, matrices or algebraic objects in general. After you study linear algebra, you should be able to understand the following concepts: systems of linear equations, Gaussian elimination, vector spaces, linear independence, spanning sets, bases, linear transformations, kernel, range, rank-nullity theorem, determinants, eigenvalues and eigenvectors, inner product spaces, canonical forms like diagonalization, Jordan normal form, etc.
Mathematical Analysis (estimated time: 6 months - 9 months)
Mathematical analysis is something that combines your understanding of most of the topics from the subjects above. It helps you understand calculus better. It helps you understand probability theory from a rigorous point of view. And it helps you understand why many of our numerical methods work. After you study mathematical analysis, you should be able to understand the following topics: the field of real numbers, open sets, closed sets, limit points, convergence, compactness, connectedness, completeness, metric spaces, differentiation, integration, Taylor's theorem, functional spaces, Jacobian matrix, Riemann integration, a bit of measure theory, etc.