The transmission system operator (TSO) and the distribution system operator (DSO) need to plan and operate together to cope with the increasing penetration of distributed energy resources (DERs). Optimization of both systems is required to reduce the global cost. But direct co-optimization of distribution and transmission grids for large systems is still computationally out of reach, especially when long-term prospective studies are carried out. Also, decomposition-based methods are either long or imprecise. This paper presents a tractable method based on the estimation of the distributed flexibility potential that captures the distribution grid characteristics. This method requires a fine definition of the flexibility the DSO could offer to the TSO. In radial distribution grids, the amount of available flexibility for the transmission grid is thus determined, which results in bounds for the TSO-DSO power exchanges over a given time horizon.