在AG Grid中，可以通过自定义列过滤器组件来限制用户在输入框中输入的字符数。下面是一个示例代码：

首先，创建一个名为CustomTextFilterComponent的自定义列过滤器组件，继承自AgFrameworkComponent：

import { Component } from '@angular/core';
import { IFilterParams, RowNode, AgFrameworkComponent } from 'ag-grid-community';

@Component({
  selector: 'app-custom-text-filter',
  template: `
    
  `
})
export class CustomTextFilterComponent implements AgFrameworkComponent {
  private params: IFilterParams;
  public filterText: string = '';

  agInit(params: IFilterParams): void {
    this.params = params;
  }

  onInput(): void {
    if (this.filterText.length > 10) { // 限制输入字符数为10
      this.filterText = this.filterText.substr(0, 10);
    }
    this.params.filterChangedCallback();
  }

  isFilterActive(): boolean {
    return this.filterText !== '';
  }

  doesFilterPass(params: any): boolean {
    const cellValue = params.value;
    return cellValue.toLowerCase().indexOf(this.filterText.toLowerCase()) >= 0;
  }

  getModel(): any {
    return { filter: this.filterText };
  }

  setModel(model: any): void {
    this.filterText = model ? model.filter : '';
  }

  afterGuiAttached(): void {}
}

接下来，将自定义列过滤器组件应用到AG Grid中的列定义中。假设你正在使用Angular，可以在组件中的columnDefs中定义列，并在filter属性中指定自定义过滤器组件：

import { Component } from '@angular/core';
import { ColDef } from 'ag-grid-community';

@Component({
  selector: 'app-grid',
  template: `
    
  `
})
export class GridComponent {
  public rowData: any[];
  public columnDefs: ColDef[];

  constructor() {
    this.rowData = [
      { name: 'John', age: 25 },
      { name: 'Jane', age: 30 },
      { name: 'Bob', age: 35 },
    ];

    this.columnDefs = [
      { field: 'name' },
      { field: 'age', filter: 'agTextColumnFilter', filterParams: { filterOptions: ['equals', 'notEquals'], filterComponent: 'customTextFilterComponent' } }
    ];
  }
}

最后，确保将自定义列过滤器组件添加到模块的declarations中：

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { AgGridModule } from 'ag-grid-angular';

import { GridComponent } from './grid.component';
import { CustomTextFilterComponent } from './custom-text-filter.component';

@NgModule({
  imports: [BrowserModule, AgGridModule.withComponents([CustomTextFilterComponent])],
  declarations: [GridComponent, CustomTextFilterComponent],
  bootstrap: [GridComponent]
})
export class AppModule {}

通过以上步骤，你就可以在AG Grid的列过滤器中限制用户输入的字符数。在示例代码中，限制了输入字符数为10，你可以根据需要进行调整。