APAR's covered conductor page cites self-supporting pole spans of 60 to 70 metres without a messenger conductor. This is a typical figure for the product, not a design rule.
The span a particular line can use depends on the conductor type and size, the installed tension, the permitted sag, ground clearance and the wind and ice loading assumed for the route. A line with an ACSR or AL59 ACS core, which carries more of the mechanical load in the steel or high-strength strands, can be designed for a different span from one with an all-aluminium alloy core. The covering adds weight and wind area, and the line designer must include both.
The conductor options and their trade-offs are compared in APAR's guide to covered conductor core options. The final span, sag and tension should be confirmed by the utility's line designer against the tender specification.